How to: Connect two batteries in parallel

Since this article was published I have received a lot of questions about connecting batteries. How To:Connect two batteries in parallel – Part 2 answers the questions asked the most.

Like most things there is a right way and a wrong way of doing it and one that I receive emails about is how to connect two batteries in parallel and get even more people finding the site by asking the question on Google. So here is a quick “How To” guide with some explanations on the right way and the wrong way.

Most people who want to connect two batteries together are trying to expand the battery capacity of their existing setup. One thing to remember, if you are going to install a second battery, you are going to have to start with two new, identical batteries. Same make, Amp hour (Ah) rating and if you can the same manufacture date. Connecting batteries with different Ah ratings or from different manufactures is not a good idea as it can/will lead to one battery trying to charge the other (due to the internal forward and reverse resistance being different) and can lead to all sorts of other problems.

So now we have two new identical batteries how do we go about connecting the two batteries together? Well the most obvious is to simply connect a new battery to the original battery using some cable and new battery terminal clamps like this….

The WRONG way to connect two batteries in parallel

OK, that will work. Hang on though lets have a closer look.

Battery A is the primary battery and Battery B is our newly installed secondary battery. Now when we put the whole system on load… starting the engine for instance, there will be a slight voltage drop across the two cables linking the two batteries together… lets say it’s 0.5 volts on each link or bridge cable, that’s one volt in total. So Battery A will always supply more power than Battery B as battery B’s apparent voltage is always less than Battery A because of the volt drop. Now over a period of time, Battery A will always be ‘used’ slightly more than Battery B so at some point Battery A will have cycled more and be ‘aging’ faster than Battery B and it will eventually lead to problems and will need replacing. But remember what I said earlier, you should always use batteries from the same manufacturer with the same rating and manufacture date… well that still applies and now you will be replacing Battery B that is probably still working OK.

The other side of this is charging. We will still have the voltage drop – although it will be smaller as the charging current is a lot less than the cranking current for starting your engine, lets call it 1/4 volt (0.25 volts) on each cable, so now the engine is running and the batteries are charging .. but Battery A gets the full charging voltage – 14.0 volts for example but Battery B only gets 13.0 volts, so not really enough. So now we are in a situation where Battery A is doing more work and Battery B is not getting charged properly. So what’s the solution?

Well, this isn’t going to cost you much….. just connect them slightly differently!

The CORRECT way to connect two batteries in parallel

By taking the electrical feed from the second battery we now even up ‘the pack’ electrically. When we start our engine now, Battery A has no volt drop to earth and 0.5 volts to the output…. and Battery B has 0.5 volt drop to earth and no volt drop to the output. So simply by moving one connection we have evened out the volt drop to both batteries. Now the same happens in reverse when charging. Battery A has a volt drop on the +Ve side and Battery B has a volt drop on the -Ve side. So we are now charging both batteries identically.

A few notes on installing a second battery

As I said earlier, always use identical batteries – same make, same Ah rating and same manufacture date if possible.

Before installing the batteries, take the time to charge them up individually using a smart charger so that when you link them together they will both have identical charges – you will ensure that you get most out of your batteries this way.

Always try to use identical size cable or larger as the existing cables connecting your old battery.

Always try to keep the link or bridge cables the same length and route them through the same hole in any metal bodywork. This will stop the chance of any elevated/quasi DC fields and ramping DC fields, especially with cables carrying 100 Amps upwards (inverters etc.) There are also other considerations when switching on/off high DC loads too. It’s how the professionals in the marine and aviation industry do it so that’s how I’d recommend you do it.

NEVER connect both batteries earth connections to the chassis of the vehicle and rely on the body of the vehicle as the electrical path . Only ever link them to each other and have ONE connection the chassis or bodywork.

All installations are different. It may be that instead of moving the positive lead to the new battery (B) is is better to leave that where it is and move the negative (earth) to the new battery instead. Just remember one lead from one battery, the other lead from the other battery. Take time to plan before committing to anything.

NEVER cut a few of the strands off the cable to try to get two heavy-duty cables in a battery terminal designed for one cable. There are specialist battery terminals designed to take two cables out there… check out the on-line specialist stores.

As you are probably doing this because you need more capacity, it is worth checking the existing heavy-duty cables… it would be a good time to think about if the existing stuff needs upgrading too.

If you want to install a second battery in your 4 x 4 as you are installing a winch, this is not the way to do it. I’ll cover that in a future “How To”

Be Safe…

There is a huge amount of energy stored in these batteries, if you short one out it will go BANG… if you connect two together the wrong way… it still goes BANG but a lot LOUDER! Seriously, if you have any doubts, get an Auto Electrician to do it. Getting it wrong will seriously hurt you.

If you found this useful, I’d appreciate it if you would click on the rating star do-hicky at the top of the page, thanks.

UPDATE: If you are thinking of installing a second battery in your vehicle, do you actually need to connect them directly parallel to make one big battery? If you don’t need pure starting power (cranking amps) from your battery, there may be other options available that would give you split charging and the ability to combine them if required. Have a read through “Relay, VSR, SCR… what’s the difference?” to see if there is a better option that would suit your needs.



Connecting Two Batteries in Parallel
Connecting Two Batteries in Parallel

I have attached a PDF drawing (Below) of the above drawing that you can down load and print out with the wiring diagrams above. How to connect two batteries in parallel 01

Further Reading:

This article by a fellow caravan writer and noted author  Collyn Rivers – “Connecting Batteries for more power” is well worth a read if you are planning on extending the battery capacity of your Caravan, Motorhome or RV.

How To:Connect two batteries in parallel – Part 2 answers the most common questions I have been asked.

Understanding Cable and Cable Sizes – When undertaking an electrical project for your caravan or motorhome one key consideration is what type and size of cable to use. Selecting a cable that is too small for the task and you might risk melting the cable insulation or damaging equipment due to voltage drop.

UPDATE – More info can be found on my blog here…. Overland Vehicle Electrics and Other Stuff…


Copyright © 2011 – 2020 Simon P Barlow – All rights reserved

Protected By Copyscape

246 thoughts on “How to: Connect two batteries in parallel”

  1. Gyula Babik said:

    Hi there,
    Slightly off topic bit hope you can help. I while ago I built our campervan and this page was very helpful to connect 2 leisure batteries in parallel, everything works as it should. I’m planning to add an inverter to my setup and something made me thinking. The inverter has a power rating of 2kw and they even supplied the cables (20mm) with the terminals crimped already. Great I thought it’s an easy fit but then I remembered that the 2 leisure battery is in different locations (approx 2 meter apart) and I can’t change the cable connecting the two together.. If I remember correctly I used 10mm cables (I’ll have to double check).
    Would the inverter pull the same amount of Amp from both battery or would it be half/half? Obviously if it’s the first then the leisure batteries joining cable would become undersized. Hope my question make sense?
    Thanks in advance.

    • Hi
      Generally the inverter will try to draw equally from both batteries, all it ‘sees’ really is one big battery. However, how the batteries are connected together and the length and size of the cables connecting them will have an effect on how the inverter draws current and usually one of the batteries will supply a little more than the other.

      To minimise this imbalance its always best to connect the inverter across the two batteries, that is the +ve (positive) lead to the inverter should come from the +ve terminal of one battery (battery A) and the -ve (negative) lead from the inverter connect to the -ve terminal of the the second battery (battery B).

      In theory as the inverter is connected across the two batteries as each battery should supply around 50% of the current demand, the interconnect can be rated smaller. In reality the size of battery interconnect cables should be rated for the maximum current load likely to be put on the battery bank. So in your case a 2Kw inverter could draw 167 Amps (Power / Volts = Current) 2000 / 12 = 166.6 Amps) It will be a bit more as the inverter is not 100% efficient so add 10 to 15% for safety. The reasoning behind this is if one of the batteries had a cell failure you probably would not know and the remaining operating battery would then supply the current demand of the inverter.

      It might be a bit more work, but personally I’d rewire the battery bank to make sure ALL cables are rated for the anticipated max current draw of the inverter.

      This article I wrote a few weeks ago might be of use…

  2. Hi Simon,
    Help please. How do l find a reliable, qualified auto electrician (preferably in London but within the M25) to check whether my second battery has been correctly installed.
    I would really appreciate hearing from you. Googling is a stab in the dark.
    Best wishes,

  3. Hello Simon , I have a negative and positive Bus Bar where do i put the Bus bar in the above layout (picture please). Thanks regards Tim

  4. Thanks for the diagram just starting to convert to van was helpfull first time doing installion do you need to put fuse on any battery installed thanks

    • yes a fuse is required…. rated for the cable size connecting to your battery. I would always advise rating the fuse lower than the max current the cable is rated for to be safe.

  5. I am setting up three x 120amp AGM batteries. I understand that all batteries need to be linked. All are going to a fuse block with appropriate fuses. I am using correct cable from positive of battery 1 to the fuse block and correct cable from negative of battery 3 to fuse block.

    When connecting battery 1 to battery 2 and battery 2 to battery 3, do I need to connect a fuse between each battery and if so how should this be done and what size fuse

    • If the batteries are all in the same compartment and are linked with very short cables, then fuses between them are not really required. You only need to fuse when batteries are remote from each other and cables linking them pass through bulkheads.

      Fuse rating should always be less than the cable rating.

  6. Hi,

    Can I connect 2 x AGM Auxillary batteries (housed in battery boxes in my trailer) via Anderson Plugs to make them operate in parallel?


  7. Ray smith said:

    Hi. I have 2 identical batteries. Not parallel connected. One runs just a fridge via the main van splitter. The other is charged via solar. I’d like to connect them so the fridge can get more power from the second battery.
    Do you have a suggestion.
    Thank you, Ray

    • Hi Ray
      Just parallel them up…. both will be charged with the solar and both will run the fridge. As long as you don’t run an inverter you can keep the wiring simple and just use the same gauge wire as is currently connecting your main battery tot he van fuse box.

  8. Michael Murphy said:

    Thank you very helpful I’m going to parallel connect to 100 amp batteries AGM deep cycle you have given me confidence I know what I’m doing now thank you

  9. Hi Simon, some great information thank you very much.
    A question if I may: I am using 4 x 130ah batteries in parralel (connected as you have recommended) if I now wire in a 2000/4000w inverter I now have to upgrade my battery wires to match the inverter. I.e. 35 mm2 or would you recommend higher?
    Also does the negative to ground have to be bigger in size than 35mm or the same ?
    Kind regards

    • Hi Stuart
      A 4000w inverter will pull in the region of 340 Amps on max load so you need to calculate the cable size based on the current AND length. For that sort of load I would want to keep the cables (+ve and -ve) to the inverter under 1 metre in length, including the fuse for the inverter.

      All the battery interlink cables should be sized to take the same current.

      The cable that links the battery bank to the chassis can be smaller in cross section than the -ve cable to the inverter as long as it is not smaller than the cable(s) feeding the non inverter loads and more importantly the main fuse rating.

      Not all cable is equal. Check with the supplier on the manufacturers current rating for the cross section you have selected.

  10. Charles M Conway said:

    I will check how I have set my batteries up in Parallel. But I believe I have them connected correctly. My problem is that they drop to 50% overnight. The only thing running is the Fridge (mostly running on Propane. and the Radio that is off but still has a light on. something is wrong I believe since I’m drawing so much power with new batteries. any idea’s? could you respond to my email please

    • Hi
      The only real way to find out what parasitic current draw you have is by measuring the current draw for each circuit. You don’t state what Ah capacity you batteries are but as they are new, did you have the same power drain on your old batteries? If not then something has changed with the installation of the new batteries.

      You can check current drain by simply measuring volt drop across each fuse…

  11. Gerard Vallely said:

    I would be grateful for your help.

    I have 2 x 100 amp Leisure battery’s in parallel – what fuse should be in the circuit. The wiring diagram shows 1 x 100 amp battery with a 20 amp fuse in series, but the reality is that the 2 battery’s have a 30 amp fuse in series. I would be grateful if you could clarify for me.

    The same idiot replaced the 15 amp Fridge, 20 amp Leisure Battery and 20 amp vehicle battery fuses with 30 Amps. These on my Autotrail tracker are located in the engine bay. I have now replaced the 30 amp with the designated fuses – 15, and 2 x 20 Amps.

    I am basing my missing fuse size on the information shown on the wiring diagram for my Sargent PSU 2007. My moho is a Autotrail Tracker EKS 2005 registration.

    Again I would be grateful for your guidance.

    Thank you in advance, Gerard

    • The fuse is designed to protect the wiring… the fuse rating should always be less than the current rating of the cable. With one battery, if the wiring diagram shows that it should be a 20 Amp, then even with two batteries, the fuse should still be 20 Amps unless you have replaced the existing wiring from your battery with a larger cross section (don’t forget this will also apply to the negative (neutral cable as well)

      20 Amp is usually fine in a standard Sargent installation in a Motorhome unless additional devices have been installed. Adding an additional battery only really adds duration to your system in this case. Even a solar charger should not affect this as it would have it’s own connection to the battery and separate fuse.

      • gerard4rox said:

        Thank you for that information. I suspect it was some genius who thought higher rated fuses give better protection.


  12. Hi Simon

    Is it best to have a ground your house batteries to the body/ chassis or to keep your house batteries and vehicle electrics separate by not grounding to the body or chassis?


    • Hi Terry
      I’m a bit old school with my thinking on this. Personally I would keep the 12 volt electrics separate (running +ve and -ve cables to each device) and only ground the leisure/house batteries to the chassis at one point at the battery location. This would allow me to fully isolate the system by removing one ground link between the battery and chassis to aid fault finding etc. It also means I’m not relying on any chassis ground paths for equipment to work. It provides the advantages of both systems as you can simply change between one and the other by simply disconnecting one link.

      I had a chap contact me he was having issues with the rock lights and light bars mounted on his roof rack not working… they were OK when installed but started to dim a bit after a period of time. He installed a roof tent and everything seemed to work OK, then the 12 volt sockets and light int he tent stopped working and his roof rack lights resumed having issues. He did some testing and could not find anything. He emailed me asking for advice… after a couple of email exchanges we discovered that the aluminium roof rack had contact corrosion where it mounted to the vehicle and he had just used the roof rack fram contact tot he vehicle as the return path ground. Installing a separate earth cured all his issues. The conclusion of this is not to rely on the chassis or frame to provide a reliable earth return path.

  13. Christy Muldowney said:

    We learn something new every day, very helpful I honestly didn’t know there is a wrong way to connect them in parallel thanks for the info.
    Regards Christy.

  14. Hi, I’m connecting 2 x 125ah lithium batteries in a camper trailer and the supplier has recommended installing a 100 amp manual reset circuit beaker between the two batteries for isolation during storage and safety purposes. This is in addition to a 100 amp breaker between the battery bank and the other breakers. Do you have an opinion on this? I’m thinking it may be overkill and will result in voltage difference issues when reconnecting after storage.

    • HI, personally I think that it’s pointless. As after any period of time disconnected you would need to bring each individual battery up to the same state of charge as the other before ‘flipping the switch” to connect them. I would keep all batteries linked in a bank and treat them as one big storage unit with just an out going breaker.

      There is a case for having individual breakers on each battery…. but this is usually reserved for connecting upwards of 5 batteries to a common busbar as found in marine setups. It also requires a next level of safety and battery management to operate.

  15. Thank you for the info regarding parallel linking two batteries. My question is how important is it to have the same manufacturers as the new battery i just installed is no longer available so cant get a perfect match. the best i can match is 12v and 130Ah ?

    • Hi Paul
      Really I’d hold off putting in a second battery until you can get a matched pair. However if you need to do it right now, match the Ah size and chemistry (don’t put a AGM with a standard Lead Acid wet cell for example). What you need to consider is that you will affect both batteries with a mis-matched pair. Charging will be un-balanced as will discharging. Inevitably this will shorten the life of both batteries. You may want to think about buying a matched pair and selling the existing battery…. or maybe make a 12 volt emergency power pack. I know of one caravaner that did this and made a stand-by battery pack he could plug into his motor mover just in case an extended stay off-grid meant he didn’t have enough power to use the motor mover.

  16. Can I charge a mobility scooter with twin batteries 55ah each with a 1000 W inverter using the scooters 2amp charger thanks mark

  17. After hooking up my 2 batteries I still go to parks where I hook up to there 30 amp service is this a problem for the new batteries I don’t always dry camp

  18. What a top man giving such superb information for the good of others…. And me as i was about to do it wrong

  19. Would it be incorrect and/or less efficient to hook a load up using a splice between the two batteries in parallel? So, positive to positive and negative to negative between the batteries as the diagram shows, but splicing the load wires into the dead center of those + and – wires connecting the batteries together?

    • Hi Jeff
      Yes you can do it that way, not with a splice though but by connecting each battery to a common positive and common negative connection. it’s usually known as s Bus Bar battery bank. The losses are usually slightly higher and the cost slightly more. If you do this it would be prudent to fuse each battery close to the battery terminal before the cable goes to the bus bar as these cables tend to be longer to reach the bus bar mounting point.

      About six batteries is the nominal cost/efficiency change over between interconnected batteries and moving on to a Bus Bar system.

  20. Hi Simon.
    I’m embarking (up the wrong tree?) on the conversion of a bus into a motorhome. Early days just yet, but I’m giving myself a headache thinking about leisure batteries and the charging thereof. One of the first things I’m going to do is install the 240v mains hookup – this will make using power tools easier. Then I’ll be turning my attention to leisure batteries, so that over the winter months I’ll be able to run the 12v diesel heater that I’m going to install and work in there without suffering from hypothermia. I’m planning to keep the ‘home’ part of the motorhome as mainly 12 volts as there are plenty of 12v appliances available. The ‘motor’ part, being a bus, is 24 volts.
    My question: how would a split charge system work, given that the vehicle’s alternator will be providing 24 volts and my leisure batteries (2 X 120ah AGM) will presumably require 12 volts.
    Am I overthinking this, or is there a blindingly obvious way to do it?

    • If I were doing it…. I opt for a 24 volt leisure battery bank so that I could use a 24 volt inverter and for 12 volts… use a 24v to 12v converter. Plenty of these around as they are common in marine and truck applications.

      Using a 24 volt battery bank is more efficient and DC chargers 24v to 24v to charge the battery bank are available, again from the marine industry. Check out Sterling Power and Victron Energy for some of their 24 volt equipment.

      A big advantage using 24 volt on a bus conversion is you don’t have to be mindful of the leisure battery location from the engine as the voltage losses are less on longer cable runs. Also the cable diameter is less for a given load when using 24 volts over 12 volts.

      2 x 120ah batteries at 12 volts gives 240ah…. but 2 x 12 volts in series sill only gives you 120ah but you draw less current for a given load, so it all balances out.

      Given a choice of a 12 volt system or 24 volt system with a 24 to 12 converter… I’d go 24 volts every time.

      Sounds like a great project. I’ve just finished watching on YouTube a Dutch chap convert a double deck coach into a fantastic motorhome for his family.

  21. Can I have two 200 amp alternators charging the batteries on my land rover for winch operation.

    • Hi Harry
      Yes, twin alternators is quite common, especially in the boating world. However it is not quite as simple as connecting two alternators together. You need an special unit that can combine the outputs of the two alternators without them conflicting. Take a look at for more info. Depending on what the alternator set up is you can go up to 400 Amps charging if you have the battery capacity. Most of Sterlings Alternator Battery chargers can be used with twin alternator set-up’s.

  22. Electrician said:

    DC doesn’t cause Eddy Currents.

  23. Hi Simon,

    Thanks for such a helpful article!
    I’m a newbie when it comes to electrics, so would really appreciate some advice please..
    I have a VW T5. I am planning to fit 2 solar panels (160w each installed in series), to a 60A MPPT charge controller, to 2 new & identical 12v 110AH deep cycle leisure batteries (Installed in parallel)

    (It will be a stand-alone system, as it was seeming too complicated to sort ‘split charge system’ with blue motion/stop-start etc.. Plus, I don’t ever intend needing electric hookup)

    I will be running a fridge (waeco crx 50), a small water pump/tap (dometic sink/hob unit), a handful of led lights & the occasional charging of an iPad/iPhone/MacBook…
    I think my system will be sufficient for my needs & appropriately connected, but would appreciate clarification, please..

    In relation to the batteries; what cable would you recommend?
    Will the system need earthing (& if so, where would you recommend..?)
    What is the best way to wire the system from the batteries?
    What size fuses would I need & where would you recommend their placement?

    Sorry for so many questions!
    I know they’re quite basic, (& I have an idea of how to do it) but would appreciate an expert eye before I start please…

    Many thanks


  24. Tony Gallagher said:

    Hello Simon
    I have two 12v 90Ah leisure batteries connected in parallel as you prescribed, I also have a 12v 250Ah leisure battery that I would like to add to the existing two to increase capacity etc. all three are the same brand good quality batteries.

    My question is are there problems with this combination and if so is there a remedy to the problem ?

    Kind Regards
    Tony Gallagher

    • Hi Tony
      You could add the larger battery to the two existing ones and run OK like that without any issues. You will of course loose some life expectancy of the smaller batteries over a period of time and recharging cycles.

      Having a bank with a total of 480Ah would lean towards upgrading the charging system so that the bank didn’t have problems with slow charging… I’d be looking for a charger with around 60 Amps output. You want to be able to bulk charge relatively quickly then open as much the equalising and float charging to preserve the batteries.

      If I had that set up, I might lean towards how many marine setups are done and have two separate banks and a changeover switch between the two… there are battery master switches that have “Off – Bank 1 – Bank 2 – Both” options.

      • Morning Simon
        Thank you for an unexpected quick reply, you obviously treat Sunday like I do never switch off.

        From your reply I think I will use the 250Ah separately, connected to my inverter to prevent the issue you raised with a change over switch from my solar charger .

        What do you think ?

        Many Thanks fill your coffee cup !

      • That would work…. you can always look out for a solar charger that has the ability to handle two battery banks as an upgrade in the future.

  25. Thanks a lot for this very informative post, I was able to reconnect my both batteries following your instructions and I have much better output and charging now.

  26. Glad I checked this out as l was just going to buy another battery. Now after reading your info will stop for a moment and work out a new plan & cost. thank you regards Chris

  27. Trevor Holbrook said:

    Hi Simon

    Couldn’t agree more with your Grandfather.
    My moto is Up to a Standard, not down to a cost.

    kind regards

  28. Trevor Holbrook said:

    Hi Simon

    I am a complete newbie regarding campervan electrics(or any other kind of electrics for that matter)
    But i am good at spotting someone who knows right from wrong.

    I have a 2018 VW T6 transporter (campervan converted).it has a Euro 6 engine with stop start.
    A Varta 7po 915 105 a 75ah AGM battery

    On the charging it has a CB 516 Switching battery charger

    A Redarc BCDC1240D 3 Stage battery charger

    A Multicell Leisure Battery SFL-110 (Lead acid Sealed)

    A 100 w solar panel (fitted to roof)

    For habitaion heating/hot water I have a Webasto Thermo Top Evo Diesel Heating Kit

    A 12v dometic Compressor Fridge

    LED lighting. Water pump. I don’t have a TV. I have a 150w inverter (12v Plug) for Laptop battery charging. I have USB charging points for my iphone, & electric toothbrush.

    I do a lot of wild camping. & only plug into 240v when at home.
    If i am wild camped for more than one day, & a night, the leisure battery indicator drops to about 11.2v , & i have to run the vehicle engine,which immediately shows a charge of 13.6v.
    I run the engine for about 15- 20 mins then switch off the engine, & indicator shows 12.6v

    My questions are, first, have i got the right leisure battery, second would i benefit from having a second battery installed( i have space right next to the existing).

    Kind Regards
    Trevor Holbrook

    • Hi Trevor
      It seems like you have a pretty good set up there. Yes you have the right leisure battery and normal charging is pretty well covered.

      But….. the issue is with the compressor fridge. Nothing wrong with the fridge, it’s a great fridge from a good reputable manufacturer. That said you really need a minimum of 200W of solar just to keep up with the fridge’s requirements over a 24 hour period. (Remember you know have daylight hours to collect via solar what the fridge takes in 24 hours). Ideally 300 watts and you can put a little more back in to the leisure battery than the fridge takes. You could add a second battery that would at least keep you out off grid a bit longer… but then it would take twice as long to charge when you ‘plugged in’ or ran the engine.

      I’d look at adding another 100 watts of solar initially (ideally 200) and then think about an additional battery as the next step.

      • Trevor Holbrook said:

        Hi Simon
        Thank you very much for the prompt(very impressed) reply, & for the information.
        Can you recommend a good Solar panel to meet my needs?
        Kind Regards

      • Hi Trevor
        Basically they all seem to be similar…. I’d look for a suitable sized one that has a reasonable power density (how many W per square metre) and from a reputable make. If you are happy with the one you have, go for the same manufacturer. Obviously if you find one that is a lot cheaper than similar spec panels from other sources, then maybe it’s too cheap. My grandfather only had two sayings…”Owt cheap is dear”…. then usually followed up with “buy cheap, buy twice”

  29. Great thread Simon! I have a caravan with one AGM 100 ah battery, I would like to add another AGM 100ah but a different brand and age , the first Battery is approx 5 y/o and going strong the one that I want to add is 18 mth old. They will be side by side about an inch away from each other. The van has a 7 stage smart charger 240v and solar, will this be an OK set up for both batteries or will I keep them separate? What size cable should I use to bridge them together? I am in Australia and have access to 4 B&S, 6B&S.

    • Hi
      In an ideal world two identical batteries are the way to go, but as you are adding a second battery that is not new it would work fine and you might as well get full use of those rather than investing in two new ones.

      Two differing batteries will always of course have some batt to batt energy transfer which over a period of time will lead to a reduction in their life, but that may only be a few charging cycles.

      Pairing two older batteries is always better than pairing one old one to a brand new one as the life of the new one always seems to be reduced almost to the life left in the old one.

      The smart charger will work fine, however I would suggest that you charge each battery separately on a smart charger before combining them as one unit to bring them both up to a good known state of charge. If you have a conditioning option on your charger that de-sulphates the plates first that would be ideal.

      The solar unit will also be fine… don’t forget however it will take a bit longer to charge two batteries over one battery even though you might have only drained the same amount of energy out of one as you do out of the pair.

      Not sure how you cable sizes work in Australia (my bad) but go for the largest cable you can. If you have an inverter… at least the same size as the battery cable fitted on that.

  30. Dimitris said:

    Hey Simon,

    Thanks for the nice and detailed explanation. I also was not aware that there was a wrong way to connect the two batteries in parallel and your explanation makes a lot of sense.

    Short question, would it not be equally okay if both the grounding and the path to the load where attached in the middle of each cable (assuming equal lentgths)? I Was planning to have two ‘gather points’ where + and – of the two batteries, the solar charge controller and the load would come together. Feels also a bit safer to put a 30A fuse on each battery. Or am I overlooking something?


    • You could do I guess…. you have the safety issue of having to disconnect two terminals to isolate the batteries. Also fuses are subject to mechanical fatigue… you could loose one and would never know as you would still be running on one battery.

      It would be non standard and something I would not recommend.

      • Dimitris said:

        Ok, first of all thanks for the ultra-fast response! You got me again, I will definitely follow your advice. Just to clarify: The MPPT controller you would connect at some point after your arrow to “electrical equipment and charging unit” (possibly in parallel with the connection the the alternator and the main battery) and the ground of all those could (or probably should?) connect to the same cable that grounds the two batteries, right? Does the length of the latter (i.e. the cable that connects Battery A to the ground) play any role?

      • I’d connect a MPPT controller via a fuse at the first termination of the battery +ve to a distribution point, and the MPPT -ve to the main -ve battery bus ground. Basically so the connections were as close to the battery bank as possible but without being connected to the battery terminals themselves. Although it seems to happen a lot, using the battery terminals as a ‘terminal block’ for all sorts of connections is not good practice. It makes fault finding harder plus all the connections are now subject to terminal corrosion.

      • Dimitris said:

        Great, thanks again for the advice. Much appreciated

  31. We have had 2 batteries connected for a good while with no problems, until this summer we went to start running the leccy and it seems the inverter is fked, scuse my French… We really don’t wanna buy a new one, but is there a way of checking the fuses hoping that it could be something inside there sorry if sounds idiotic but we can’t afford new inverter… We’ve got digital ballast and wondering if we can run leccy off of that… Id appreciate some replies if pos cos we pretty snookered with it, cheerz

    Sent from rich

    • Hi Rich
      Some inverters will not turn on if the battery input voltage is below a pre-set level. As you have had your two batteries for a number of years, they may seem to be charged up OK, but soon as any load is applied the voltage may drop below the pre-set level for the inverter. I would do a simple check and connect the inverter to your vehicle battery just to test it with a small load… say a 60 watt table lamp. This may confirm if the inverter is OK or not.

      In my experience inverters usually fail when on load. It’s worth performing this simple test first.

      The other check I’d do is a load test on your batteries. Most garages usually have a battery tester that puts a load on them for a few seconds and measures the voltage drop and could do this test for you.

      Inverters do usually have some sort of internal fuse, unfortunately most are a cheap fused link on the circuit board and is not generally user replaceable.

  32. Why same batteries I’ve seen people who
    Av ad a 110 to 120 no problems
    And is a fuses needed from the 2 batteries
    Does battery need a metal base

    • Hi Bryan

      Ideally the two batteries need to be the same Ah rating otherwise they will discharge at different rates… and recharge at different rates. The larger Ah battery will over a number of cycles tend reduce its capacity to that of the smaller battery.

      If you connect two batteries that are a few feet apart… then the +ve cable needs fusing at both ends, otherwise if it only had one fuse in the cable both sides would still be live even with a blown fuse.

      Regarding the metal base… it is usually best to sit them on a non metallic base to avoid corrosion, ideally in a moulded battery box or a battery bag.

      • Calum Shaw said:

        Hi Simon
        Could you give a bit more detail about why the larger Ah battery would reduce its capacity? The discharge/recharge rate will be proportionate to capacity, so I don’t see any problem with that.

      • Hi Calum
        There are a number of reasons, and if you search “battery bank imbalance” there are lots of articles that go into the details.

        However briefly, if both batteries are discharged to say 50%, on recharging the smaller capacity battery will reach a fully charged state first and the charged voltage of the smaller battery will shut off most smart chargers… the larger capacity battery will then never reach its full capacity and over a period of time will start to behave as the smaller capacity battery.. shortening it’s life in the process.

        Additionally, once the smaller battery becomes fully charged, the larger capacity battery will still have a lower internal resistance and the charger in trying to satisfy this will have the result of overcharging the smaller battery slightly… eventually shortening its life too.

        You will never charge the larger capacity battery at the optimal current as the smaller capacity battery will affect the output voltage of the smart charger.

        If you drain 10% capacity out the battery bank, the lower capacity battery will have an individual drain different to the larger capacity battery. As this leads to slightly differing voltages between the batteries, they will try to balance themselves to equal voltage.

        Discharge/recharge is not proportional to individual battery capacity in a bank as effectively you don’t have individual batteries. The charger only ‘sees’ a total capacity based on voltage and current.

        There are multi output smart controller chargers available for battery banks. They are designed for marine use and usually for banks of 800Ah upwards.

  33. Pat. Fenton said:

    Can I connect two 6 volt batteries and 3 12 volt batteries. And how to connect them.

    • Hi
      Technically… yes you can… just connect the two 6 volt batteries in series, then treat them as one 12 volt battery and connect the 12 volt batteries in parallel.

      However…. it’s not a good idea to put two 6 volts batteries in series with 12 volt batteries as the charging characteristics will be different and you will get the ‘pack’ unbalanced and risk not charging any of the batteries correctly and shortening their life considerably.

  34. Hi Simon, thanks for this blog… do you have any tips on connecting 3-4 batteries in parallel most effectively? All the same brand and type… mainly for having enough amps to safely power a microwave and a big inverter. I know not ideal but I couldn’t afford the Trojan batteries i’d of preferred :).

    • Hi Lee
      Having multiple batteries means potentially you have a massive amount of energy stored. Always make sure you have some sort of master battery bank disconnect and a master fuse rated below the maximum current carrying capacity of the battery bank cables.

      • Of course… I wanted this many so that any draws from a hair dryer or similar for the inverter or start up microwave draw would be catered for. Would you suggest the same method of connecting the batteries as described here? Or would there be a better option?

      • Hi Lee
        Once you start getting above three batteries consideration has to be given to installing two copper bus bars and linking the batteries to these so that you can take each battery out of service for maintenance or charging separately without disrupting the main bank…. but this requires more space and a lot more work making and installing custom bus bars. The method I outlined above is suitable for most boat, motorhome or caravan situations.

  35. John Trelease said:

    Great help thanks Simon

  36. brian Markham said:

    I have two battery chargers each with a nominal output of 27.4V at 10A. Each of these are charging two 12V batteries connected in series, the output of each of these systems are commoned to together through two blocking diodes eg. four diodes in total. the measured voltage on the common side of the output of the diode pairs is 71V. whats gone wrong I would assume the maximum voltage measured at this point would be 27.4V.
    Could you please give me some ides of what you think is wrong

  37. Hi, I didn’t understand the bit about routing both cables through the same hole in the bodywork to stop the chance of eddy currents. Aren’t eddy currents a function of AC rather than DC?



  38. Jerry harris said:


  39. If i Connect the two 12 v battery in series and i can get 24 v and the same current 180AH.
    With the same battery I can Take + and – of one single battery means the Voltage will be 12 V but How much will be the Current.
    Wheather we can take the Battery connection like This Or Not.
    Please Give me Reply

    • Hi

      If you connect two 12 volt 90Ah batteries in series you will get 24v 90Ah capacity. If you ‘tap’ off one 12 volt battery you will get 12v 90Ah capacity.

      Only if you connect two 12 volt 90Ah batteries in parallel do you get 12 volt 180Ah capacity.


    Yes Sir , I have a 2014 GMC Sierra 1500. I had to replace the original MF battery just recently and lt took quite a bit more time to extricate it out of its’ bay. And I noticed that the top of the battery had a snap on plate that housed the hookups for the negative and positive connections plus a cable going to the starter , three in all. Is it possible using your second diagram above to still hook up a dual battery setup provided that it’s same AGM gel battery of the same make-amperage- date of MFG, like you discussed in your article???

    • Hi Douglas
      I don’t see why not as long as the original cables or their replacements are of the correct size (gauge) and the battery interlink cables are sized to take the maximum current rating of the original cables.

  41. That was a great help.It was so informative.I have never see it exploded so well,especaly the connecting so you get the same out of each battery.I am putting 2 A.G.M extreme gel battery’s together 200ah in paralle.One more question.I would like your help with please.I am fitting a split charge Relay,and 400w of solar 2×200 panels on the roof of my camper van convention.When I run the wires from My Charge controller to my battery’s.Witch battery do I connect to + and – ,and same with my S.C.R.Do I connect to the battery + across from the earth.The left battery on you diagram.Hope to hear soon.Thank you for your help so fare

    • Hi Graham
      Connect the solar charger across the battery bank… so +ve on one battery and -ve on the furthest battery. You want to connect charging across the whole bank not one end if you know what I mean.

  42. I have 10 batteries and I want to connect them to a home solar system, each battery is 12V 100A.
    How do I connect all 10 batteries that I’m just getting a 24V 500A?

    • Hi
      You need two banks of 5 batteries.

      Connect 5 batteries in parallel giving you 12 volts, 500 Ah. Connect a second bank of 5 in parallel giving you another 12 volt 500 Ah bank. Once these are done, you connect these two banks in series giving you 24 volts 500Ah.

      • Continue to my question Do I connect as you said to me, it will be the best connection or can be connected in another way with me best Afekat

      • You could connect them as 5 pairs of 24 volt units… the best way can only be determined by working out the volt drop for the maximum current drawn with the selected cable size and length.

  43. Thanks m williams we have110amp 3 months old thanks for the information

  44. I have a question, I have seen a diagram of a battery connection between 2 where the POS goes to neg and then vise versa on the secondary battery , does this also work

    • Hi Andrew
      What you might have seen is the +VE of one battery connected to the -VE of another… this is known as a series connection and is used to connect two batteries to increase the voltage. For example, two 6 volt batteries connected to produce 12 volts.

      Only one side of the batteries is connected… one +VE to one -VE… the other terminals being the supply (+VE) and return (-VE) at the increased voltage.

  45. Couldn’t find the star thingy. So just posting to say excellent complete article. Thanks

  46. Hi Simon Thank You for explaining how to connect 2 batteries in parallel properly . I have read many articles on this subject and many people’s ideas is to connect them the wrong way .. I have scene 2 batteries connected this way but no explanation as to why .. Thank You

  47. Hi.

    1) Does this work only for batteries that are of the same age? i.e. if I have a two year-old battery and a four year-old battery in parallel, is this still the best way to hook them up?

    2) If I have two 12V batteries hooked up the “correct” way, but I have a two bank charger that charges both batteries at the same time, how would I hook up the four leads from the 2-bank charger?

    P.S. This is for use in a tournament fishing boat that requires both batteries to be charged in short order –
    overnight – hence the 2-bank charger set-up.

    Thanks for what you do!.

    • Hi Mark
      You can use batteries of different ages and Ah ratings but this will inevitably shorten the life of both batteries. Trying to charge two different age batteries in a battery bank will definitely shorten the life. Knowing this though will allow you to plan replacing both.

      If you have a two bank charger, personally I’d disconnect the batteries from the bank and charge then individually. Each battery because of its differing age will have a differing charge profile and trying to charge them in a bank will invariably mean one battery is not charged to the maximum it can hold.

      To make disconnecting them simple, a suitable switch installed on one of the interlink cables will allow you not only to switch off the bank but disconnect the two for charging.

      To prolong the life and get the best out of your batteries you might want to consider a smart multi stage charger such as a CTEK and once every few weeks putting each battery through a reconditioning/recovery cycle that includes a desulphation phase… its surprising how much some of these chargers can improve mid life batteries performance.

      • Simon, thanks. The two bank, marine charger charges each battery independently, giving them how much juice they need to get up to speed,

        There are two separate pairs of +- cords – one pair for each battery.

        So, while they are connected in parallel as you describe, can I just connect one set to one battery and the other set to the other battery and charge that way, as if they aren’t connected in parallel?

      • Hi Mark
        Sorry for the delay in getting back to you I’ve had to do a bit of research on bank chargers.

        Bank chargers are designed to charge individual batteries in a series wired bank… i.e two 6 volt batteries wired to produce 12 volts or two 12 volt batteries wired to produce 24 volts. A bank charger will charge each individual battery.

        When two batteries are wired in parallel all you are in effect doing is using two separate chargers to charge the bank and this could result in each part of the charger”seeing” the other and fooling it into thinking the battery has more charge than it actually does.

        A bank charger is less effective on a parallel connected battery bank.

        You will need to disconnect the bank to charge with a bank charger so the batteries are individual units or upgrade to a smart charger that is capable of charging a battery bank of the total Ah rating of your batteries.

  48. Rob mcewan said:

    Just put 2nd battery in my caravan as you have shown on your direction ,i have 140 kw ,solar panel is that sufficient, if it is when i had 1 battery we use to run out of power free camping will the 2nd battery give me additional length of power if so approximately percentage wise how much,i have 2 100amp batteries

    • Hi Rob
      So many variables here. A second battery has increased your capacity to store energy, however it can only store what the solar panel can output. So if your solar panel could not recharge your original single battery that lasted 2 days, then it will be unable to recharge two batteries.

      You need to have a solar panel that can recharge your battery in about 8 hours to full. I calculated I’d need around 300 to 400 watts of solar in the UK to be sure of recharging a 110Ah battery in all weathers based on draining it to about 60% charge.

      You have to think about how efficient your solar works as well. In summer you might have 12 hours of usable sunlight, about 5 of it will be overhead enough if your panel is mounted flat to be effective. In winter there is about 8 hours of usable light, but for the sun is low so not as efficient. Tilting panels make a huge difference.

      If possible I’d look at increasing your solar panel capacity and maybe upgrading to one of the digital solar controllers that monitor your energy usage.

      • Rob mcewan said:

        Can i connect a 2nd panel direct to the battery that is capable to follow the sun that is not attached to the roof

      • Hi Rob
        That would be my preference. Make sure it is connected to the battery via your solar panel regulator though.

        It is better to connect all your panels through the same regulator so you might need to check the regulator is rated for the total wattage of the solar panels you have connected.

        I have seen it where people have two solar regulators and still did not get the charge they were expecting, this was down to the fact one regulator was charging the battery and because of the charging voltage, the other regulator thought the battery was fully charged and turned off.

  49. Tom Stanley said:

    I would also like to add an addendum to my last question please. A friend who is a retired industrial electrician has a similar setup to mine and he has his wiring as follows- +/- solar leads from panel to controller. +/- leads from caravan(which before fitting solar went to one battery) to ‘load’ connections on controller and +/- leads from twin paralleled batteries to ‘battery’ on controller. Is this correct please?

  50. Tom Stanley said:

    Hi, Can you tell me please if wiring two batteries in parallel like you have suggested, will work on a solar charging system like I have. I have two matched 180ah batteries with a pwm controller(60amp) and two 250w solar panels wired in parallel? Thanks, Tom.

  51. Ian Sibson said:

    Hi Simon
    I want to use my portable solar panels in conjunction with my caravan fixed panel. Is it possible to do this. I ask this because sometimes we have to park in the shade so adding the portable and placing it in the sun would help charging.

    Ian Sibson

    • Hi Ian
      Yes, a lot of people do have portable panels to augment a fixed panel. Simply add an additional connector into the panel input of your solar regulator so the additional panels are in parallel to the existing panel.

  52. say if had two battery @6 volt make 12 volt then it has 300 amp each battery that would make 600 amp but how many amp hours would it have

    • Hi John
      If you add 2 x 6 volt 300 Ah batteries in series to give 12 volts you will still only have 300Ah capacity. To double the Ah capacity you would need to connect the batteries in parallel… that would give you 6 volts at 600Ah.

  53. Hi Simon,

    Wonderful explanation, thanks so much. I do have a question about connecting batteries in parallel but not into the car system. I want to connect two batteries together to sit on their own (not wired into the car’s systems at all) with an ARB fridge ‘plugged in’ to them. I plan to charge them during the day with a ‘suitcase’ solar panel using alligator clips (while the fridge is still plugged into them).

    Is it okay to just hook the batteries together with two cables (pos-pos, neg-neg) and use them like that, or do I also need a separate ground wire hooked to some metal part of the car from one of the neg terminals?

    Thanks much. Doug

    • Hi Doug
      You don’t need to connect your battery bank to the vehicle ground. However you might want to, unless you have a solar panel that can provide enough power in an 8 hour period to replace the energy taken out of the batteries over a 24 hour period you will never be able to fully charge your battery bank.
      Typical figures for running a ARB Fridge:
      ARB Current 60Ah 80Ah 100Ah
      Fridge Draw Ah
      35L 0.85 49hrs 75hrs 82hrs
      47L 0.87 48hrs 64hrs 80hrs
      60L 0.89 47hrs 63hrs 79hrs
      78L 1.07 39hrs 52hrs 65hrs

      Ideally you need solar that will be able to input energy at least 2.5 x the daily drain, therefore it might be an idea to think of having a secondary charging system that will allow you to charge the battery bank from the vehicle.

  54. Paul Winters said:

    That’s really helpful. I would have connected them the wrong way. Many thanks Paul.

  55. Hi,

    Is it not at all possible to , if there are financial difficulties, use a smaller battery then the primary car battery. I was thinking like an Alarm battery ?

    • Hi
      Alarm batteries are typically 17Ah, a leisure battery usually 100Ah, so about 1/5 capacity. Additionally they are designed for a tiny current draw over a long period, not short bursts of high current draw.
      It might be OK for LED lights for a few hours, but running the water pump and other accessories would soon drain it to the 50% state of charge level that is recommended for lead acid batteries very quickly. I would not really recommend going for an alarm battery.

  56. another question. what’s your thoughts on flooded batteries for camper van use?cost wise they are cheaper how would they hold at against Gel?

    • Hi Dino
      Personally I’d opt for Gel if I could get them in my price range, otherwise it would be wet cell, but not sealed ones. I’d like to be able to top them up as required.

      • Hello again Simon, I have a question about connection the wiring to load, how about connecting it to the opposite terminals (ones that are not connected to the power source) wouldn’t it be more efficient as they wont be connected to the same terminals where power is coming in? or wouldn’t it make a difference??

      • Hi Dino
        You could, however the convention is not to split the load and charging points. Remember when on electrical shore power the charger also acts as a regulated supply for any load, If you have the load on one side and the charing point on the other, that means the regulated power supply/charger will be supplying power through the batteries to the load and you will not get the benefits of a step charge that most modern caravan and motorhome units are capable off.

        For safety you should only connect to a single point so if you have to disconnect the battery bank you only have to do it at one point. It also aids fault finding.

  57. Hi Mate, I’m all new to this, my used camper which I purchased recently has 2X100amp 12v batteries where one needs replacing and the other doesn’t seem to have much life left in it either, so I need to replace them before I set of on my camping adventure, My question to you sir is which is a better choice of replacement? 2x100amp batteries or just the 1X200amp ? and also during winter when not in use is it best to disconnect all wires and let sit idle?

    • Hi Dino
      Personally I would go for 2 x 100Ah batteries and in winter remove them and keep them at home and use a good quality multi stage charger such as a C-Tek or similar to keep them conditioned ready for the next season.

      • curiosity always gets the better of me. may I ask why 2x100amp as opposed to 1x200amp pls??

      • Hi
        If you get a cell go down in one big battery, you have to replace it, with two batteries you only have to replace one battery as long as the batteries have been looked after, there should not be too much of a miss match.

        It’s sometimes also easier to locate two physically smaller batteries.

        One big 200Ah battery is often more expensive than two 100Ah batteries due to the difference in production volume.

  58. Thank you for your guidance

  59. Thanks very useful info will be watching for further posts

  60. Simon,

    Some good info when you get to read it all. The only problem I have with the parallel hook ups is it is easy to end up losing both batteries, if they don’t have independant monitors. It is quite common (in australia at least) for 1 battery to drop a cell (common on rough roads) and by the time it is found the second has either overcharged to death, or been so badly damaged they both need replacing. I’ve a system that isloates the 2 batteries so that each independantly charges but then connects off charge to give a greater ‘Ah’ bank for draw. Helps limit the problem and makes the ‘Same age/same type’ not such a big deal.


  61. trevor willis said:

    excellent read it all very useful

  62. Robert atkinson said:

    Hi Simon, I have been reading some of your posts and they are really helpful. I was wondering if you had any advice on what to was hoping to do in my camper.
    Basically I was hoping to fit 2 small leisure battery’s in my conversion (simply because I was hoping they would fit somewhere more descreet like under the swivel seats (to avoid filling up cupboard or under bed space and as yet I haven’t found anything much good for extra battery’s under the bonnet of a vw T5). Doesthis sound reasonable? Also does this mean I would need different power level of battery’s or can they just be joined in parallel as you’ve described previously. thanks in advance. Rob

  63. U did not explained about fuse rating while hooking up battery u should do dat

    • Hi
      The fuse rating will depend on what size cable you use. Only ever rate a fuse to what the cables are rated for.

      If the cables are rated for 100Amp…. that is the MAXIMUM size fuse you should use.

      I would always down rate the fuse to be safe, so with cables that are rated for 100 Amps, I’d use a 80 Amp fuse.

  64. Great article. Thanks. Clear and concise.

    I was just wondering. If you somehow flattened the batteries that are install in parallel (in the correct way). What would the procedure be to jump start the battery with some jump leads?

    Would you put the jump leads onto the actual terminals that are connected to the car wiring and earth?



    • Hi Matt
      The procedure would still be the same for jump starting… always connect the +ve battery terminals first with the jump lead, then connect the -ve jump lead to an earth point on the vehicle, never the -ve battery terminal as this could create a spark near the battery which could ignite any gasses emanating from the battery.
      Also something to consider now is that with modern vehicle electronics there are special jump leads available that protect sensitive electronics from potential damage from jump starting.

  65. Hi im glad I found this as I thought this was how it should be done. I do have one query though.
    I’ve just bought 2 150 ah batts and wired them as you suggest. BUT, what always confuses me is regarding the earth. Say for example I have a solar reg which is running a charge to the batts, from e.g solar panel, do I connect it to the battery negative at the battery or do I earth the battery to the chassis and then just bolt my solar reg negative to the chassis somewhere too? Or at the same earth point?

    • Hi Matt
      In an ideal setup all the earth connections would be on the battery, however this is not practical. The best way is to take the earth from the battery bank to a common point and earth everything there. Don’t rely on the chassis to provide earth return paths for any devices, it makes fault finding hard and you have to be sure all the earthing points on the chassis are in good condition.

  66. Bob Terrett said:

    Hi Simon
    I recently purchased a new caravan to which I have fitted a second battery. The new battery is identical to the original (which is only a month or two old). Because of space restrictions the two batteries are located 3.5 metres apart. I have connected them in parallel as per your recommendation with + take-off from battery A and – take-off from battery B. For the connecting wires between the two batteries I used 8B&S cable. Is that acceptable?

    • Hi Bob
      I’m not too familiar with the AU wire sizes but I think 8B&S rating is 72 Amps, so as long as you don’t intend pulling more that 70 Amps continuous from you battery bank is should be OK.

  67. Frank Bath said:

    Thanks Simon I was looking at those the other day. I’m glad I didn’t confuse you to much with kit ( most people wouldn’t have a clue!)
    The Ambulance is actually being converted to a mobile first aid unit, however still have blues and twos! As well as being used as a ‘camper’ for me when covering scout camps (trolley cot is really comfortable but narrow!) I do have the current 3 batteries being charged from alternator with a cut off circuit so that the starter battery won’t drain if the other 2 are in use (currently there is 1 leisure battery, starter battery and a small battery to run radio coms, so I am adding a second leisure battery to improve power output) I have a 240 volt back up charger that will recondition the batteries and constantly monitor their performance.
    Which I have turned on constantly at home, but can also be pluged in whilst on camps if power available and I am also adding more solar panels on the roof so I am hoping that eventually most power will be solar. Only thing is not sure if I should use second leisure battery or just see if solar panels will be the way forward?



  68. Frank Bath said:

    Hi Simon

    This is really useful. I am planning on adding a second battery to my 05 Master ambulance in order to add 240v sockets. I have both a solar charging system and an external 240v. I do have 1 question regarding power inverters. I need to be able to charge and power an ecg, BP and cardiac monitor as well as mobile phones, notebook computer. etc (not all at once) what kind of wattage do I need? If anyone else can help it will be appreciated


    • Hi Frank
      My wife is a Clinical Perfusionist so I do have a bit of background knowledge regarding the kit you are wanting to power. I would recommend a good quality Pure Sine Wave inverter of about 2500/3000 watts. I would seriously look at Sterling Power products for your application.
      I would also look at a DC to DC charging system to optimally charge up your battery bank while the engine is running. For your application it has to work 100% every time.

  69. allan rolfe said:

    i have a 100ah battery in my caravan and want to add another 100ah battery,if I connect them the way you have shown would it be ok. going to use portable sola panels.

  70. phil parkin said:

    just a quick extra question with the wiring of the two batteries. Would it matter if took the load off the opposite terminals, it would reduce my wire length by 12″ ?

  71. phil parkin said:

    Hi Simon – I have a motor home with a single battery – I have just added/connected a solar panel to maintain this battery, The solar has a twin battery output, so I now want to add a ‘spare’ pair of batteries (wired as per your excellent advice)(I had a pair of brand new units salvaged off a mobility scooter, so it seemed a good idea)! I intend these to be connected to the solar for charging, but to be used completely independently from the original single battery with an output socket for USB and cigarette lighter adaptor for tv etc (as shown at website link below).
    Could you advise on wiring – do I need to connect to vehicle earth at all, since it will effectively be a ‘stand alone’ system? And how do I connect the 3 components on my output panel? one cable split at the unit or individual cables from battery to each of the 3 components.
    Thanks in advance – and thanks for an excellent blog
    regards – Phil P

    • Hi Phil
      Yes, you will need to connect the earths for a stand alone system… you do need to follow any advice on the solar charger for connecting a second battery though.
      For the devices… I’d use one 6mm cable to a fuse distribution strip, then one cable from each fuse to each separate device.

      • phil parkin said:

        Thanks Simon – thank you so much for taking the time and trouble to help us all!

  72. JOHN VALLE said:

    Hi Simon,

    thank you for writing this, I am in the process of doing a BIG stereo on my motorcycle (Harley Davidson) my plan is to run one or two EXTRA batteries besides the main battery that starts and runs the bike. I was thinking of running an isolator type unit for boats to separate the two power supply,

    like this one, the goal is to always have power to start the bike and when parked use the power of the extra battery or batteries to run the stereo for as long as I want, and when is time to go the bike will always start because I have not touched the power of the main battery….hope that makes sense.

    do you see a problem in running them in parallel?


    John V.

    • Hi John
      I don’t see any issues with what you propose. The only thing I would check out though is the alternator output. I think it’s a relatively low output and it would be worth checking to see how long it would take to charge the additional batteries from 50%.

  73. Hi I have a 2 year old numax 110ah battery and a new numax 110ah battery I want to put together is this wrong to do so all so what size cables do I need and will the cables that are in the caravan now be okay to take the power of two batterys ?

    • Hi Karl
      You should be OK with these two batteries. The cable in the caravan will be fine… all you are doing is extending the amount of power you are storing not increasing the amount of power going through the caravan wiring. However, if you are installing an inverter, you need to size the cables between the two batteries and between the batteries and inverter to take the maximum current that the inverter is capable of drawing.

  74. tanya curran said:

    would the wiring be the same if you are connecting 3 batteries

  75. woodseaves said:

    Thanks for your advice. As a matter of interest there have been a number of incidents that upon reflection might be attributable to the wiring error. When the solar panel starts to charge there is sometimes a clicking noise from the fuse box usually accompanied by the 240v alarm. Do you think they may be connected?

    • Hi
      You didn’t give any details about the wiring error, so its difficult to know. However, one thing that I have picked up on with some solar panel installations is over voltage upsetting the inbuilt chargers.

      With some chargers if it ‘sees’ the battery voltage is too high, it switches the battery out of circuit and continues to supply regulated 12 volts to the domestic electrical services and periodically checks the leisure battery voltage to see if it has fallen within its parameters. This “could” be the relay you are hearing. I’m not sure about the alarm unless it too is upset by over voltage. Personally I’d get the solar regulator checked. On some later motor homes and caravans there is an input for solar panels to be connected directly to the installed power supply/charger unit to get round this issue. It would be worth checking with the manufacturer.

      • Hi again,
        The wiring error was as in your first example, the error that wasn’t an error if you like. Anyhow I carried out the rewire and although it is only day two all the noises have stopped. I don’t know how to tell the garage that it was wired up wrong or should I just leave it?
        Thanks for all your help and support.


      • Hi
        Glad it seems to be all sorted out now.

        Its always tricky deciding if to report back on something, however it might be worth letting them know about the problem and how you solved it as it might be something they hadn’t come across before and will aid them correcting the issue in the future.

  76. I have a 2 year old motorhome that has 2 leisure batteries in parallel. They were fitted new 2 years ago and wired incorrectly. Is it to late to wire correctly or do I need to replace the batteries?

    • Hi
      The batteries will be fine, correct the wiring and they will go on for a few more years yet. If you have a multistage smart charger its always a good idea to disconnect them and charge them individually every 12 months to make sure you can get the best performance and maximise their life.

  77. My power inverter says each battery should be grounded separate why?

    • Hi Jason
      I can’t think of any reason why it would say that. The only thing that crossed my mind was they might have had past experience where the grounding of second batteries was not up to standard. I have come across situations where the positive cable was sized correctly from the batteries, but the earth (ground) cable was way too small.

  78. Jeff Parsons said:

    Hi Simon, I have been reading these posts and your responses with great interest. I would just like to confirm my set up is wired correctly? I have a Motor Home.

    I have two 120ah leisures batteries wired in parallel as per your No2 diagram. I have wired the original battery loom which is wired between my EC325 sergeants power unit and the two batteries to Negative on battery 1 and the positive on battery 2. I also have an inverter which I have also wired to the same two terminals ( Neg Batt 1, Poss Batt 2) is this the best wiring? Kind regards Jeff

  79. Hi Simon, you make it very simple and logical, thank you. I have a slightly different problem. or an extension of the 2 battery parallel connection. My situation is a marine, connecting house batteries for my yacht. My concern is that I have 3 house batteries, and need to connect them in parallel, is it possible to connect 3 batteries this way so that each battery has an equal amount of drain and recharge?

    I would appreciate your help. Thank You!

  80. Thanks
    I was thinking there was a method like this
    Tank you

  81. Norrie Crawford. said:

    Hi Simon,
    I have two new 12V batteries in parallel connected to a 1000W inverter. Whenever I turn on our TV it will run for about 15 mins and then the power will go off and the alarm on the inverter goes indicating low battery power. Can you explain this fault?


    • Hi Norrie
      If the TV is around 200 watts when operating, this equates to a current draw of 16.5 amps at 12 volts not including what the inverter requires to operate. You didn’t state the Ah (Ampere Hour) rating of the batteries but assuming they are around 100 Ah each, from fully charged, new condition batteries, we can calculate that 250 watts (that’s to include the power the inverter requires) is about 41Ah, so to run two 100 Ah batteries down to 50% you should get around 2 to 2 1/4 hours of viewing on your TV before the battery low voltage alarm goes off.

      However, if the batteries are in less than tip top condition, or they are not matched, this will reduce your viewing time. It might be worth investing in a smart charger such as one from the C-Tek range to fully recondition the batteries (do this individually) and maximise their charging.

      It is also worth checking that all the cable terminations are in good condition and the cable used is of the minimum size required. For a 1000 watt inverter I would use 16mm2 cable and keep the cable length as short as possible to reduce the voltage drop.

      The only other thing it could be is that one or both of your batteries are ‘aged’ and not keeping their charge or fully recharging. You can usually get them checked at a local battery supply outlet.

  82. Rahul sharms said:

    hi simon
    i got to know how to connect two batteries in parallel..
    but i have a question..
    can this parallel connection be performed across an invertor having capacity of holding single battery.??
    i.e. for series combination of two batteries we require an invertor of 1600 watt.,
    while 800watt inverter can hold single battery.
    so can 800 watt inverter hold two batteries with parallel combination instead of series……????????

    • No.

      Two 12 volt batteries in parallel give you 12 volts.

      Two 12 volt batteries in series give you 24 volts.

      If the input rating on the inverter – no matter what the output wattage says 12 volts, you must only connect two batteries in parallel. Connecting them in series will seriously damage the inverter.


  83. h hemsley. said:

    hello simon
    is it ok to run a 12v tv connected to a separate leisure battery via 12v cable with crocodile clips going to a female cigarette socket connecter thanks?
    with regards harry.

  84. This makes no sense why don’t you just put the earth at battery B? the fact is once they are in parallel it doesn’t matter which way you connect the battery because there is no battery A and no battery B they become one battery. the fact is you should always use the exact same batteries when putting them is series or parallel and there will always be a voltage drop between and batteries that are connected by wire. the longer the run of wire the more resistance so the more of a voltage drop you will get.

    • Hi Rob
      The fact that the two (or more) batteries are in parallel doesn’t make them one battery. If you take the feed and return from just one battery you get differential voltage drops between the batteries on both charge and discharge. Moving either the earth or +ve to the opposite end of the battery bank means the differential voltage drop is eliminated and the voltage drop is the same for both (all) batteries. Generally this is how commercial battery banks are connected, even when using large bus bars.

  85. Hi Simon,
    What an interesting blog. I wish to maintain charge on 2 Camping 12 volt 7ah batteries by means of a Benton maintenance charger. I believe I must connect them in parallel. They are both fitted with Cigarette lighter female sockets. Can I plug in a split unit to one, a coupler with two male connectors from one split to the other battery and plug the charger into the second split socket. This puts your outside element, the charger, between the two batteries. Looking at the advice given, it would seem that I should charge both fully first. I then want to just leave on charge until I next need them?

  86. I have a 1000VA/800W/12V inverter.
    I want to connect two 12V/120Ah batteries. Should i connect these in series or in parallel to increase the backup.
    Series will double the voltage and parallel configuration will double the current. Which i need?
    Kindly do reply me

    • Hi
      The only way to connect the two batteries for your application is in parallel. You want to keep the voltage the same – 12 volts and double the current available. Your two batteries will then be the same as a single 12 volt 240 Ah battery.


      • Hi Simon,

        I have a 800 VA inverter and this time i connected three 75 AMPs batteries in parallel. 12v inverter has two charge modes, regular giving ~12 amps and fast giving 16 amps. now since i have 3 batteries the charging current is divided equally and each battery is getting under ~4amp. i also know that fast charging affects battery life too but in order to get the same charge time i either need 36 amps charge current from my inverter which is not possible since its fixed either at 12 or 16 amp or my charge time of all 3 batteries has reduced by 1/3 compared to same inverter charging a single battery. the only option i am left with seems like is to put on fast charge mode which will give 16 amps to all three batteries. This will be 5.3 amps per battery which is still less than 12 amps but will increase the charge time. this however will still be almost 1/2 less than the time it would have taken to charge a single battery.

        i hope my logic is correct and putting the fast charge will not damage any battery since its already significantly low than original 12 amps required to charge one battery. Any further suggestions on how to get back charge time for all 3 batteries if inverter can give max 16 amps and no more?



      • Hi
        Having three batteries in parallel does prove slightly more problematical when charging. Personally I would set the charging to the higher rate for the bank. However, once a month or so I’d switch out each battery and charge it individually on a smart charger such as a C-Tek to equalise the bank. You can get battery post terminals that are quick disconnect. (link below) These should always be installed on the negative terminal and would allow you to disconnect one battery at a time easily.

  87. Alastair Hibbert said:

    A very useful article, thank you. In other guidance I have seen, fuses are recommended close to each positive terminal. Would fuses be a useful precaution and if so how large would they need to be to connect 2 X 110Ah batteries running lights and a TV in a motor home? I cannot see my onboard equipment drawing any more than 4-8 Amps.
    I am also a bit confused about how heavy the connections cable should be. The existing cable looks about 4mm2 – would similar cable be sufficient to connect the batteries or does it have to be heavier?

    • Hi Alistair
      I would recommend fuses are installed at the +ve end of each cable. The correct method of sizing a fuse it to rate it at the maximum or below the maximum rating of the cable. The fuses are there to protect the cables from overload. If your current battery is connected by a 4mm2 cable, then when adding a second battery it would be OK to use the same size cable.

      However, if the second battery is located some distance from the first (on the opposite side of the motorhome or at the rear for example) it would be usual to increase the size of the interlink cables – in your case to 6mm2 to help reduce voltage drop.

      The nominal capacity for automotive use cable is:-

      4mm2 cable is rated at 27 Amps – so fuse at 20 Amps
      6mm2 cable is rated at 42 Amps – so fuse at 30 Amps
      10mm2 cable is rated at 60 Amps – so fuse at 50 Amps

      Once you get above 30 Amps I would generally not recommend using blade type fuses, rather go for the screw mount fuses – these can often be found in the audio sections of auto-parts stores for people upgrading car audio equipment.

      Hope this helps.


      • Alastair Hibbert said:

        Thank you, Simon.

        My present battery is fused at 40Amps on a 4mm2 cable in my Bessacarr MH! Cable lengths are fairly short as batteries are end on to each other. I was planning to fit in-line continental type fuses with screw connections. I cannot find 20Amp fuses so I was thinking of putting 16Amp fuses in. From what you say, that would seem to be OK?

        Thanks again.


      • Hi AListair

        Just double check the cable size… it might be 6mm2 with thin insulation. This is sometimes used if the cable isn’t running through lots of bodywork or is inside flexible conduit. A 40 Amp fuse is rated too high for 4mm2 cable, 30 Amps would do at a pinch.

        I suspect 16 Amps will be fine, it is doubtful that the charging current even with dual 110Ah batteries will reach 16 Amps, however it might be worth trying to get 20 Amp or I believe 25 Amp fuses are available from some distributers.



  88. Ive drawn a circuit diagram on paint to make things easier.
    This is how I have planned to upgrade my system. The yellow lines is what I think you have proposed.
    everything else is currently installed and ready to terminate.
    which would be the best idea do you think?


  89. Thanks Simon for your reply, there’s multiple diagrams out there and I want to get it right.

    I see in the existing setup cabling (1 aux battery) that a battery earth has been taken from the negative post on the aux battery to a stud on wheel arch next to it… is this a sufficient earth? I’m unsure whether I need to run an additional neutral back to the starting battery or not.
    From the starting battery I have a 35mm cable to (inline isolator first) the aux battery no1. Ive decided to run the link cables between the new aux batteries in 16mm. I could upgrade these to 35mm if necessary and run a neutral from aux battery 2 back to the neutral of the starter battery or the grounding post next to aux battery no1.

    Its a small toyota hiace Hitop 4wd campervan with 2 lights and all I want to do is run the lights on an evening and charge my phone and laptop. there is also a small water pump I never use.

    From what you have said I think I need to upgrade the 16mms link cables to 35mm and run an additional neutral (earth) back from aux battery no2 to the ground stud on the wheel arch adjacent to aux battery no1.

    Or run an earth from aux battery no2 back to the starter battery.

    When you say ‘distribution piont’ do you mean the stud on the wheel arch or the starter battery

    It would be difficult to move the power circuit neutrals and the inverters neutral onto auxilary battery no2s post as would involve so major rewiring. So If I connect like we’ve said above this will balance the system? I just need to know which would be sufficient/suitable. Id rather have a balanced system to be honest. Sorry again for more questions.. I can send you a wiring diagram of what I propose to do if thats easier, email me.



    • Hi Harry
      I’ll email you a drawing tomorrow (it’s nearly midnight here) based on your information. I would think that 16mm2 cable should be alright – I’ll work out the voltage drop to be sure.



  90. Hi, I found this info very useful. I think I would have connected my inverter up on the 1st auxilary battery, which now seems the wrong way to do it.
    Im upgrading my auxilary battery in my campervan, and also adding a 2nd aux battery on the other side of the van. (unfortunately the joining cables are about 2m between aux batteries, i’m not sure if this is ok :S)
    All my circuits and inverter come from the housing where the 1st battery sits, so after reading this it seems I will have to rewire so that the inverter and the loads come off the positive of the 2nd aux battery, or reroute my cabling from my starter battery. Ive installed a 140w solar panel too and I planned to connect the controller onto the second aux battery. Does it matter how this connected? Should the controller be connected across aux battery no1 (+ terminal) and aux battery no2 (- terminal) so that the charge coming from the controller is distributed evenly or doesn’t it matter? I presume the same theory applies?
    Thanks for the great tip and if you could answer this question if you can understand my setup from the description I would most appreciate it.
    Also good tip about charging your batteries first and keeping cable lengths the same.. I like it 😀
    Ive done several large battery bank installations but these small tips never occurred to me so thanks.
    It delays my setup by a few days but I don’t mind if this gives my installation more longevity.
    Harry – Industrial Electrician – New Zealand

    • Hi Harry
      The further apart the two batteries are, the more important it becomes to link them in the correct way. If the batteries are adjacent, the affects of voltage drop on current draw and recharging are minimised if the interlink cables are only a few inches or centimetres. However once the distance becomes greater, and the interlink cables are longer then the voltage drop will be greater.

      From what I understand, your battery A will be one side of the vehicle and battery B on the other side, with maybe 1.81 to 2 metres of cable linking them. In your case I would definitely wire the systems so that all current draw and charging circuits came from the +ve of Battery A and the return path would be to the -ve terminal of battery B to minimise any imbalance due to voltage drop.

      For ease of wiring, you could interconnect the batteries with a suitable heavy grade cable (starter motor size) then run an additional main earth cable from the -ve of battery B back to your distribution point for connecting all the earth’s together. This way the two batteries will be connected as a common unit and any voltage drop on the earth cable from your earth distribution point back to the batteries will be common to both batteries and they should remain equalised. This would also mitigate having to run multiple earths from various points back to the second battery (this is sometimes how they do it with house batteries on boats when they are located in obscure sections of the hull).

      Hope this helps


  91. Hi, I have a dual battery system in my vehicle with a deep cycle battery and solar panel on a camper trailer . Our vehicles alt charges the the dual battery from which a cable runs to Anderson plug connection to our camper battery while it is still connected to the solar panel . Is this a flawed system ie should I have solar disconnected whilst travelling or leave solar and disconnect vehicle alt.?
    Cheers mark

    • Hi Mark
      Sorry for the delay in replying but your comment got caught in my spam filter.

      Your set up should be fine. While the engine is running the battery in the camper trailer will be topped up by the vehicle and the regulator on the solar panel will see a higher voltage and just assume the battery is charged. When the engine is turned off, the solar panel regulator will respond and if the battery needs additional charge, it will provide it.

      The additional benefit of this set up is you have the option to let the solar panel charge the vehicle battery if the trailer is connected via the anderson plug which is handy if stuck in the outback with a flat vehicle battery.

  92. Very useful information helped me understand how to wire my battery’s correctly thank you

  93. Hi Simon,

    Thanks for this excellent article. A quick question from you, I own a car that runs on two 6v batteries in series that struggle to turn over the large engine that’s attached to it. I have run two small 12v batteries in parallel before in a similar car (negative earth) with no problems. The difference here is that this car runs a dynamo, not an alternator. Do you think that would introduce any issues and preclude me from running the same setup again?

    Many thanks,

    • Hi Neil

      As far as the regulator that handles the output from the dynamo is concerned all it sees is one 12 volt battery… either two 12 volt’s in parallel or two 6 volt’s in series it’s all the same.

      If one cell in either of the two 6 volts batteries in series has a problem, then the bank as a whole is affected.. both for starting and charging ether from a dynamo or an alternator. If one cell of either of the two 12 volt batteries has a problem, then the effect on the bank as a whole is minimised for starting and charging… as the unaffected battery will receive a full charge from either a dynamo or an alternator.

      There are two downsides to using a dynamo. The first is the the lower output per RPM compared to an alternator and the second is the regulator, which can be a bit temperamental sometimes and lead to undercharging or ‘cooking’ batteries.

      Personally I would run two 12 volt batteries in parallel… because of the cost compared to 6 volt units and reliability. I have a friend that runs a classic MG that I gave exactly the same advice to and he’s been happy for the last few years with the setup.

      Hope that helps


      PS.. I have heard that there is a company that produces an electronic dynamo regulator in a classic Lucas housing that is supposed to sort a lot of the issues out. I don’t have any details, but maybe a search on Google would turn something up.

      • Many thanks, Simon. I ran two 12v’s in parallel over a decade ago when I had my MG BGT V8 with no problems. I think the problem in starting the car with the originals is the low CCA (280 ish) versus two decent modern 12v (600+) – ? The switch to the batteries in parallel and a few other changes made the car a delight to start compared to how it originally was. Today, gear reduction starters make a massive difference too, but I need to keep the starter motor and dynamo for concours competition. The issue I have at the moment is with my positive earth Austin Healey 100 as it really struggles to turn over the 2.6l engine on the dual six volts. It’s newly restored so it’s not a grounding issue. Many thanks again for the advice, and I will look up that dynamo regulator too.

      • Hi Neil
        I think the electronic regulator was built in an original bakelite Lucas regulator box so it would look like an original item, I even think the wiring connections were the same.


  94. Hi, didn’t know about 2 different ah batteries in parallel. I have a 130ah and a 105ah deep cycle agm, same brand in my camper trailer. Is it best to use them seperate, eg one powering my fridge and the other to run lights and water pump rather then in parallel? Do I need to run an earth lead to the chassis for both batteries? Thanks Ash

    • Hi Ash
      You can use them in parallel but you are not going to get the best out of them over a period of time. I would use a good quality charger (C-TEK or similar) to charge them individually to make sure that they both received the correct charge. As the smaller capacity battery will charge quicker, the larger capacity battery may not receive a full charge each time when being charged in parallel.

      There is the option of using one battery as the main battery, then switching over to the second battery when required. Maybe a small solar panel to keep the standby battery ‘topped off’ until you switch over.


      • Thanks for that, do you need an earth to the chassis of the trailer to run my accessories? parallel and/or single set up batteries? I also have a 160 watt solar panel for charging when camping. Still deciding whether to join them or leave them apart.

      • Hi Ash
        In a caravan it is usual to keep the 12 volt earths separate from the chassis as the caravan chassis is connected to the earth for the 240 volt system for safety. I’m not sure if you have a 240 volt EHU system on your camper trailer but I wouldn’t think you need an earth to the chassis.

        Most problems with 12 volt systems are due to using the chassis for an earth return path… usually corroded earth bonding points. It much better to use a proper bit of wire as the earth return path I think.

        If you have a 160 watt panel I think what I’d do is keep the batteries connected as a parallel bank and after each trip when I got home disconnect them and charge them separately with a smart charger to make sure each battery was in tip top condition. While away have them as a bank and top the bank up with the solar panel, which at 160 watts is quite a handy size.


      • Sounds good thanks for helping me with that decision to join them, does it matter what battery is the main one, ie 130 for battery A and the 105 for battery B?

      • Hi Ash
        It won’t make any difference.


  95. You have some good points here. I was looking for information on how to make the physical connections for batteries in parallel since battery posts are designed to take just one cable clamp. Ended up with some good points on same type of battery etc. and pre-charging, thanks for that.

  96. After I posted it I realised that was the case! Having re read your reply and the batteries are in Series not parallel so m cable needs to be upto the 100amp so the 7.5mm cable or above will suffice
    Thanks again

    • Hi Alan
      If they are in series, it’s a bit lighter on cable.

      For example a 3000 watt inverter running on 24 volts (2 x 12 volts batteries in series) the expected current draw is 3000/24 = 125 Amps.

      Which makes it a bit easier cabling. The loss due to voltage drop will also be less.


  97. Hi
    Nobody has asked yet what size cable would you say for connect in the 12v 100amp batteries together?

    • Hi Alan

      A lot depends on the quality of the cable, number of strands and length when determining the cross section of cable to use so it’s not a straight forward choice. I would always go for a higher strand count over a lower one.

      But as a rule of thumb for a cable spec of 196/0.40 (Thats 196 strands of 0.40mm dia) un-tinned:
      CSA = 16mm2 – 110 Amps (Outside diameter 7.5mm)
      CSA = 25mm2 – 170 Amps (Outside diameter 9.8mm)
      CSA = 40mm2 – 300 Amps (Outside diameter 11.8mm)
      CSA = 60mm2 – 415 Amps (Outside diameter 14.5mm)


      • Thanks for that it’s two 100amp batteries so I will look out for
        12mm min cable

      • Hi Alan

        The battery capacity doesn’t really have any bearing on the cable size.
        If you were connecting a 3000 watt inverter, this could draw 250 Amps (P/V = I), so you would need to use cable rated for 250 Amps.

        A 2000 watt inverter would draw 167 Amps

        A 1500 watt inverter would draw 125 Amps

        A 1000 watt inverter would draw 83 Amps

        So the cable should be sized for the maximum that you would draw from the battery bank.


  98. Tony Hogg said:

    Hi Simon Could I fit a Solar Panel to keep a Battery connected to an inverter charged up .Cheers Tony.

    • Hi Tony

      Simple answer… Yes. Ideal solution in fact.

      Depending on the size (wattage) of the solar panel though you might need a separate regulator. Most of the small wattage panels.. up to 40 watts or so, have a simple regulator built in. Once you go above that wattage more often than not you will need a separate solar panel regulator.

      Depending on the usage of the inverter – how much current you draw for a period of time, you will need to size the panels so that you can replace the energy taken from the battery in a reasonable time. This is a handy site with a lot of info…



  99. Hi,
    My boat engine has 2 starting batteries,both brand new.
    There is a selector switch to select either battery or both in parallel.
    Are there any negative effects to using both new batteries simultaneously to start my engine.

    • Hi
      As long as both batteries are charged equally using them together to start the engine will reduce the load on each battery and prolong the the life. As there will be less voltage drop when cranking the engine, the starting should be easier too.

      • Thanks mate
        Appreciate the quick response

        What if one battery was weaker than the other one or if one battery had a problem like a dead cell or a short circuit.
        What problems would I encounter.

      • Hi
        If one battery had an issue – dead cell for example then there would not be any benefit from starting with the batteries in parallel. It also might take you longer to discover that one battery had a problem unless the charging system could monitor each battery individually. Twin starting batteries benefit from intelligent split chargers that can monitor both batteries and provide balanced charging.

        To get the best performance and life from batteries it is always worth investing in a good charging system. These guy’s hve some excellent products for marine and leisure use or search for “alternator to battery split chargers”.


      • Many thanks

  100. in the words of Ian Drury,you don’t arf meet some clever people,dontcha! Great help and advice thanks.

  101. Tim Cooper said:

    Hi Simon,
    Thanks for prompt reply.
    It all makes sense now.

  102. Tim Cooper said:

    Hi Simon,
    Very interesting.
    A couple of questions if that’s ok.
    How does it work if the two batteries are in a caravan, where you do not put an earth wire to a chassis.
    Also, I am thinking of adding another battery to my present one, where I also have wires from my solar panel regulator connected to the battery.
    So if I add another battery, do I just leave the regulator wired just to the one battery?

    • Hi Tim
      The connections are the same as shown in the second drawing above,but the chassis connection shown on battery A connects to the negative side of any circuits instead.

      If you install a second battery in the caravan, link the two together as shown in drawing two, then connect the positive from your solar regulator to the positive on battery B and the negative from your solar regulator to the negative on battery A, that way any voltage drops will be equal between the two batteries.

      Like wise, the existing connections between the caravan and existing battery need to be moved, so that the positive is always from one battery and the negative from the other. Again this equalises any voltage drop between the two batteries, especially if you have a high current devise like a motor mover.

      Hope that helps


      • Ray Bray said:

        Hello Simon,

        I have been given your post on batteries from the Chausson Owners Group on Facebook and have read this with interest, I did have two questions if I may.

        1. The negative side of drawing 2 I hope is the Negative lead from the the charging/discharging circuit?
        2. My second question is about the solar panel connection, but reading the above, you have answered this so, is that the same for Motorhome batteries, Positive lead from the solar controller to Batt B and the negative lead to the negative on Batt A? and therefore as their is a strap cable between both batteries the solar panel will charge both batteries concurrently.

        Very many thanks for the article and ALL the info, it really does make for great reading.

        Ray Bray

  103. Hi Simon,
    I’m a complete noob and hoping you can help me out. I have 2x 12volt 100ah batteries @20hour rating and connected in parallel powering 120 watts of lighting. The amperage draw is about 10 amps. As I understand it I should limit the amperage draw to 5amps per battery because of something called peukerts law. My question is… if I connected my batteries in series and up the voltage to 24 would I be able to draw more power from the batteries ie. 24volts x 10amps = 240watts without it affecting the rated 100ah @ 24volts?

    • Hi Sam
      The answer to your question is…. it’s probably easier to work it out, so here goes… (if I can remember my college physics lessons!)

      Herr Peukert was a German chap that in the late 1800’s decided his batteries weren’t holding their charge or so it seemed and he discovered that batteries don’t like being discharged and the faster you discharge them the less they give out… or “Peukert’s law expresses mathematically that as the rate of discharge increases, the available capacity of that battery decreases”, very simply put of course. However after lots of head scratching he came up with this:


      (k is a power)

      H is the rated discharge time
      C is the rated capacity at the discharge rate (Ah)
      I is the discharge current
      t is the actual time to discharge the battery in hours
      K is the Peukert constant.

      The value of k is normally between 1.1 and 1.3. It can range from 1.05 – 1.15 for AGM batteries, 1.1-1.25 for Gel, and 1.2-1.6 for Flooded Batteries

      I’m going to assume k = 1 in both cases for simplicity.

      OK, so in parallel you have effectively a 200Ah battery @ 12 volts and a 120 watt load (10 Amps)

      so t=H(C/IH) or 20 x (200/(10 x 20) = 20 hours

      in series you have a 100Ah battery @24 volts and a 240 watt load (10 Amps)

      so t=H(C/IH) or 20 x (100/(10 x 20) = 10 hours

      You can see that running parallel at 12 volts for a 120 watt load gives a result of 20 but if you run in series with a 240 watt load you get a result of 10. As you have doubled the load at 24 volts you have effectively halved the run time. If you keep the load the same for both series and parallel options you end up with the same figure.

      Without knowing the exact specifics of the batteries to get an accurate number for ‘k’ and working out transmission losses in the cable and joints due to resistance and the specifics of the load, in general for the same load in Watts it should not make much difference.

      Running two batteries in parallel gives you some resilience if one battery fails although the lights will go dimmer quicker, running in series with the same failure means the lights would go out.

      I ‘think’ I have got all that lot right, but I’m happy to be corrected by my peers. I’m not a battery expert by any means!

      Hope that helps


  104. Rob Downie said:

    Care should be taken when paralleling batteries. If one battery develops a problem, a very large current can pass from the good battery to the dud, resulting in an explosion.

    • Hi Rob
      Thanks for your comment.

      Batteries can store massive amounts of energy and if released in a short period can and do cause explosions. However I have not heard or read any reports of a battery failure (cell failure) in a bank causing an explosion, can you expand it further or if you have some links about it you could post, I think that would be helpful to everyone reading.

      My take on it is if two batteries are in parallel and one battery has a cell failure due to plate buckling or collapse, the potential of the battery would drop to around 10 volts and the remaining battery in the bank would still have a potential of just over 12 volts, so in theory the reverse potential across the failed cell would be just over 2 volts (with no charger connected). As the cell failure is usually over a period of time, this should be noticed by the efficiency of the bank as a whole reducing. Cell failure in one battery usually results in the remaining 5 cells being overcharged and ‘gassing’ excessively and the smell is usually a good warning something is not quite right with the bank as a whole.

      The most likely cause of cell failure is unsuitable charging (unless it’s a manufacturing fault, a cheap battery or constantly being over discharged) and charging a single battery is more likely to suffer from unsuitable charging than a pair or bank of batteries in parallel as there are more cells to take up any overcharge problems.

      As I said I’ve not heard of or read any reports of any batteries exploding because of a cell failure in one of the batteries in a bank, but there are several cases of incidents caused by people connecting or disconnecting chargers while the charger is turned on and the resulting spark igniting the hydrogen gas given off in the charging process.



      • Noted your comment re. disconnecting a charger without switching off potentially creating a spark causing hydrogen to ignite/explode. By mistake I did that last week not really appreciating the consequences. Now I know and will be extra careful in future.

  105. Outstanding !Thank you my friend. very understandable.

  106. Simon thank you so much for your time it explains loads you have been very helpful

    regards John

  107. Hi Simon thank you for your reply
    what i meant was keep the 4 batteries together as 500amps
    then get 2 more batteries separate from the 4 batteries to run the dc motor coupled to the alternator that would put out 215amps @ 700rpm i would slow the motor down as it spins at 3200rpm so would less power to run the motor
    the 3kw inverter would not be running all night just to keep 2 small 750w oil rads on low thermostat control just to keep the chill off when rads off the inverter would go back to sleep mode then kick in when needed again i thought by using this high output alternator i thought i could keep the batteries topped up at night only then fully charge in the day while out and about just trying to get away from using a small generator thank for your help

    • Hi John

      Two 750 watt heaters would draw 125 Amps from your battery pack, that’s assuming the inverter is 100% efficient (It’s more likely to be around 80% so your current draw would be probably be closer to 150 Amps (around 1800 watts) So your 500Ah battery pack won’t last too long if you only take it down to 50% capacity to stop the batteries from having a very short life span. The inverter is drawing a similar current as a vehicle starter motor from the batteries.

      The motor/alternator set up… if the alternator gives out a max of 215 Amps at 12 volts… that’s 2580 Watts…. assuming 100% efficiency again, you would have to put in 2580 watts of energy from the motor no matter how you geared them (Energy in = Energy Out minus Energy Lost)… which means you would be drawing 215 Amps out of the two batteries supplying the motor. If they are 125Ah batteries totalling 250 Ah they will run down pretty quickly. Assume 80% efficiency and you will be drawing over 250 Amps.

      Quoting your motor size from an earlier post – Your 12 volt 29 Amp (12 x 29=348 watts) 250 watt motor is only capable of delivering a maximum 250 watts of mechanical energy, therefore you will only ever get a maximum of 250 watts of energy out of the alternator no matter what size it is. (assuming 100% efficiency again)

      It would be more efficient to just add the additional batteries to the existing four rather than try to use these as an energy source to recharge your main bank.

      Something else to consider is a solar panel array to recharge your battery bank, but to put back the amount of energy you take out running the heaters for one hour over the course of a night will require a big setup to recover that amount of energy in daylight hours.

      As the battery pack increases in size, the charging period will increase too given a limited energy source. So what you are suggesting assuming the inverter is 80% efficient and the motor/alternator is 80% efficient means to run your heaters for one hour (over the course of an evening) off the battery pack, to put back that energy using the motor you have would require running your motor/generator for over 4 hours each day. (which I think means you would need 4 or 6 125AH batteries)

      Each time you convert energy from one form to another you always get a loss. To go from stored chemical energy in the battery, to electrical, to a different voltage electrical, to heat is very inefficient.

      To be honest, I think you have to look for another form of heating… or go for a petrol/LPG generator which will need to be run for at least a couple of hours per day powering an intelligent charger.


      • Hi,
        Isn’t easier to use a 12DC/220AC inverter to run the heater rather than using 12v?

      • Hi
        You could use an inverter to go from 12VDC to 220VAC… however inverters are not 100% efficient… usually about 75 to 80% there will be losses.

        So if you wanted to run a 150 watt heater at 12 volts that’s 12.5 Amps, but to run a 150 watt heater at 220 volts through an inverter from the same 12 volt source you would be drawing around 15.5 amps from the 12 volt supply.

        Converting energy from one level to another always results in losses.

  108. Hi Simon
    i have a 12v 3kw sine wave inverter with sleep mode and 4 x 125 amp batteries now what i would like advise if possible i have a second alternator 12v 215amp hr at only 700rpm it is a monster alternator i was thinking of running a 12v 29amp 250w motor to run the alternator geared to low torque on and off from the 4 batteries to top them up and not having to run the engine any advice please any or am i just wasting power from the 4 batteries


    • If I understand you correctly….
      You have four 125Ah batteries you want to use to run a 3Kw inverter. You also want to use these four batteries to power a 250 watt 12 volt motor that is coupled to an alternator and use that to recharge the four batteries.

      Every time you convert energy from one form to another (chemical to electrical, electrical to mechanical etc) there are always losses involved. So, all you would do is flatten the batteries quicker.

      • simon would i get away with two battery’s as a stand alone device to run the dc motor this would only be run 15 to 30mins each time also using a small battery charger to keep the two batteries topped up

        thank you for your reply

      • Hi John
        Using two batteries to run the motor/alternator set up to charge the remaining two batteries would loose more energy than having all four batteries in one bank. Even using a 12 volt to 12 volt charger is inefficient.

        If you need to top up the battery bank, the best way would be a small petrol generator either running a smart charger such as a C-TEK or some generators have a 12 volt battery charging output. A 2KVA generator with a charger could top up the battery bank far more efficiently.

        Google ‘inverter generators’. Honda are one of the best but expensive, Kipor, Clarke and Hyundai are reliable ones as well. “Suitcase” style inverter generators are relatively quiet and most can be converted to run on LPG if you don’t want to carry petrol.


  109. Malcolm Dockerty said:

    Hi Simon,
    If I had not read your information I would have connected my batteries the wrong way
    Thanks a million.

  110. Bob toop said:

    Hi Simon,
    I have just setup two agm batteries in my van on a solar system and did not know there was a right and wrong way to connect two batteries in parallel. Thank you very much for your information, and yes it does make sense how you have explained it.
    Thanking You Very Much
    Bob Toop

  111. Colin Snowden said:

    Hi Simon,

    In all my years, I did not know that there was a right and wrong way to connect two batteries in parallel, I have never seen this during my training days as a vehicle sparkey, nor have I seen this in any book. I must say, it all makes sence when I read what you have explained.

    Kind regards


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s