How To: Connect two batteries in parallel – Part 2


I’ve written this page to help answer some of the many questions I have been asked after the original article “How To: Connect two batteries in parallel” was published. It covers some of the practical considerations to installing battery banks in caravans, motorhomes and vehicles.

A word of caution: Leisure batteries store an incredible amount of energy and if you short them out or connect them incorrectly then this energy can be released almost instantaneously with disastrous consequences. If you have any doubts about what you are doing then seek qualified professional help.

Connecting more than one additional battery

In the first part we went through the correct way to connect an additional battery…

Practical Considerations of Battery Banks 01

A number of people asked for a drawing to show the correct way that more than one additional battery could be connected so that the voltage drops across all the batteries would be equalised and the batteries remained balanced in the bank.

Practical Considerations of Battery Banks 02

Remote Battery Location

The next question often asked is how to connect a second battery if it can’t be located next to the original battery. Installations in some motorhomes and campervans only have space for one and the second battery needed to be located remotely at the rear of the vehicle or on the opposite side.

REMEMBER: When installing a second battery in a remote location it must be sealed off from the interior of the vehicle and the battery compartment vented external to the vehicle. There are battery boxes available that do this. 

The existing set up will have all the live and neutral cables terminated near the existing leisure battery and it would take some effort to have to relocate all the earths for each individual circuit to the second battery location, there is a simple way of achieving it.

For the remote location, an additional cable is required. In the first drawing at the top of the page it shows the live cable coming from the second battery which is fine if the two batteries are adjacent. However with a remote battery this would require running two live cables through the vehicle and common sense tells us that it would be better if we only had to run one live cable. (drawing below)

Practical Considerations of Battery Banks 03

In the drawing (above) it can be seen that now we have three cables running to the remote battery. Two battery interlink cables and a main earth cable. This has now reduced the amount of live cables running through the vehicle and allowed the original earth terminal block that the auxiliary services use.

Cable size

A number of people asked about the sizes of the cables required to link the batteries together and raised the question could the cables linking the two batteries be smaller than the actual cables connecting the battery bank to the equipment. Their thought being as each battery was supplying 50% of the current the size could be reduced.

Practical Considerations of Battery Banks 04

In the drawing above the battery bank is wired using 100 Amp cable (I’m rating the cable by current rather than cross-sectional area as it’s a bit easier to follow) From the battery bank there is a 100 Amp positive and a 100 Amp negative cable. The battery interlink cables are sized at 50 Amps as that is the current between the two. We’ll ignore any fuses for the moment – more about that shortly.

Well OK it could work, however if you were using an 1200 watt inverter that put a 100 Amp load on the battery bank and a cell in battery A failed so the battery effectively had little or no output, then Battery B would continue to supply the 100 Amp load put on it by the inverter… and the interlink cables are only rated for 50 Amps, therefore something’s going to get hot!

Practical Considerations of Battery Banks 05

If you are going to demand 100 Amps from your battery bank… then use 100 Amp cable throughout the installation.

Calculate the maximum load you will ever put on your battery bank and size the cables to that load. Before finally selecting and settling on a cable size though it’s worth reading Understanding Cable and Cable Sizes as there might be a bit more involved.

Here is a rough guide to current capacity for good quality cable:-

cable size

Where to install fuses?

Convention is fuses are installed in the positive cable coming from the battery bank.

Practical Considerations of Battery Banks 06

However if there is a short in the positive battery interlink cable to ground because of the longer cable run through the vehicle due to damage or chafing, then the main fuse will not protect anything in this event.

The sensible thing would be to install a fuse on the interlink cable as well… but at which end? The conventional location is as close to the battery as possible… but there are two batteries and if the short is at mid-point one battery will still be connected.

So we have to install fuses close to each battery. Now the problem comes with what size fuse.

Practical Considerations of Battery Banks 07

Fuses are there to protect cables from overload and possible overheating and the general rule is the fuse rating should be equivalent to the current rating of the cable or lower.

Lets assume we have completed the installation in cable rated to 100 Amps. So we install fuses rated at 100 Amps at the points in the drawing above.  Disaster strikes and we get a partial or intermittent short somewhere along our positive battery interlink cable to ground. Now at the point of the short, potentially 200 Amps could flow – 100 Amps from each battery and not blow the fuses. With 200 Amps we could be welding bits of metal together. Because it’s in the middle of the battery bank there also might not be any indication that there is a problem…… until it’s too late!

Is there a solution?

Well this is what I came up with:-

Practical Considerations of Battery Banks 08

Installing a second fuse in the main neutral earth cable would protect the battery bank from any potential shorts down to the vehicle earth from the battery interlink cable. If there were a short the current would blow the fuse in the battery bank main earth cable.

IMPORTANT NOTE: There must be no connection between the battery bank and vehicle or other earths/grounds other than at the main earth terminal block for this to work!

For clarity I have removed the fuses from the drawing that were in the positive battery interlink cable. In practice it would be prudent to leave one in to protect against inadvertently shorting out while removing or installing batteries.

Installing a battery bank isolation switch

Another question that kept cropping up was based around “How can I install a battery isolator but still let the solar panels charge the batteries even when they are turned off?”

As long as the solar panel regulator is connected to the battery side of the isolator switch, then even if the isolator switch is open (the battery bank turned off) then the solar panel regulator will still be able to charge the battery bank.

Practical Considerations of Battery Banks 09

Although the isolator and cable might be rated for 100 Amps and the battery bank fuse be 100 Amps, if you are connecting the solar panel isolator with 10 Amp rated cable, then the fuse between the battery bank and regulator needs to be sized accordingly… i.e 10 Amps (remember the fuse is there to protect the cable and must never be rated higher than the cable it is protecting)

If you have found this article useful, please like, comment, subscribe.

Further Reading:

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.

Copyright © 2011 – 2016 Simon P Barlow – All rights reserved

36 thoughts on “How To: Connect two batteries in parallel – Part 2”

  1. adrian shaw said:

    what if the electric system is pms is adding a second leisure battery wired up by moving the positive to the second leisure battery first

  2. Is there any benefit in fitting a 4 way battery isolator switch in a dual battery system in RV with 2x 120ah agm + 2x 120 w solar panels, 15amp charger

    • Hi Phil
      I guess you are thinking of being able to switch between batteries and charging the out of circuit battery. In my opinion, keeping the batteries connected as a bank means reducing the discharge load on each battery. If you have one battery in circuit and discharge it to 50% then switch over to the other battery you are increasing the duty cycle of the battery. Leaving the two connected means that taking the same energy out of the two in a bank as you have done for one only discharges the pair by 25% each and charging the pair, although slower than charging one battery at a time will keep the pair as matched as possible. Also there is an element of keeping it simple reducing any losses due to terminations and the fact it is all too easy to forget to change a switch over sometimes.
      Personally, I’d keep them as one bank and charge the bank.
      Simon

  3. Many thanks Simon, great advice and website.

  4. I have a 100 watt solar panel and have 2 x100ah identical batteries I have wired up the batteries as the article and all seems ok. I have wired up the solar panels pos and neg tobattery A is that. Correct ? . although there is a regulator on the back of the panel when it appears that when the batteries are fully charged or being overchargedan alarm goes off on the control panel of my motorhome this obviously goes off when I disconnect the panel, do I need another Controller ? Regards Rob

    • Hi Rob
      Really the +ve needs to go on to one battery and the -ve from the regulator needs to go to the other battery to make sure any voltage drop is even between the two batteries.

      I would suspect the alarm is the over voltage alarm on the motorhome, although with out knowing the details it’s hard the be certain. I would check the output from the solar panel regulator to make sure it was within it’s specification max voltage. I would also check to see if there are any details listed in the motorhome manual (or with the manufacturer) on the max voltage before the over-voltage alarm is activated. On a lot of motorhomes/caravans now, the manufacturer includes a connection for solar panel regulators, it might be worth checking if there is a connection on your set up.

      If the regulator is built into the solar panel and is faulty (giving an over voltage output), I would refer back to the manufacturer/supplier of the panel.

      Simon

  5. Mike Plenty said:

    if I install a second battery will I have to move all the negative cables to the second battery?

    • Hi Mike
      The +ve leads should originate from one battery and the -ve leads should originate from the other battery in a two battery set up. It is sometimes easier to move the +ve lead if there are several -ve leads terminating on the original battery.
      Simon

  6. Hi Simon, Many thanks for your reply I will move the connection across and check with the manufacturer about the dedicated Solar panel input although I cant see one on the control panel of my Trigano T720. Rob

    • Hi Rob
      Sometimes the solar panel input is just a 2 pole connector in the wiring loom. Our Sterling Celebration caravan has one hidden away behind the charger.
      Simon

  7. great read! made my simple application even simpler.
    setting up a two battery bank. Thank you.

  8. Brilliant article, really helpful.

  9. Syd Hutchinson said:

    Fascinating article but I have a question no one has asked yet. I have 3 110Ah leisure batteries. One is 6 years old, one is 2 years old and the one is brand new unused. They are all identical Numax XV31MF. Would it be OK to parallel the 2 year old and the new together? I don’t understand why you stated the manufacture date should be the same. All batteries have been maintained with a CTEK charger.

    • Hi Syd
      Basically as soon as a battery is filled with acid at the point of manufacture or by the retailer the ageing process starts. No matter how efficiently an older battery is maintained, it will have aged compared to an younger identical battery. When you pair an older battery to a new battery generally the new battery will age quicker to match the older battery as its the older battery that will dictate the charging regime.
      This is a bit of a generalisation though as each manufacturers products behave slightly differently.

      Again another thing to note is that even though two supposedly ‘identical’ batteries from the same manufacturer but manufactured one or two years apart carry the same specification and part number, they in all probability will have some differences due to manufacturing changes and material changes made during the intervening time.

      Personally if I was setting up a new battery bank, I’d go for matched products. However, if I was just adding to an existing battery bank to extend the capacity, knowing that I may be reducing the life of the new battery slightly by adding it to an existing bank, I’d be OK with that and plan at some point in the future on upgrading the older battery when funds allow and performance drops.

  10. Syd Hutchinson said:

    Thanks Simon – Ideally another new battery to match the existing new one would be good, but that’s another £80, and then I’m left with 2 perfectly good spares. I really don’t want 4 batteries. I will try the 1 year old with the new. Hopefully they will both last several years.

  11. John Cole said:

    Thanks Simon, excellent info, looking to set up a chalet with off grid electrics and this has been a good introduction into battery ‘set ups’

  12. Very interesting read. I do have a question though.

    I’m connecting two batteries of the same voltage in parallel:

    Battery A has a relatively small capacity (I cant remember the exact number) and is flat.

    Battery B has a much higher capacity (110Ah) and is fully charged.

    When I connect these together in parallel, the 3A fuse installed between them blows. I have previously just connected these straight together with no fuse and it has ‘worked’ but I wonder what this might be doing to the batteries?

    I guess because the potential difference between the batteries initially is relatively high, and the resistance between them is low, that the initial current is high (thus blowing my fuse), but will this damage the smaller capacity batteries in absence of a fuse?

    Am I correct in thinking that once the pd between the cells has ‘balanced out’ surely they’ll discharge at an equivalent rate such that the voltage of each cell is maintained? And will just ‘look’ like a single cell of the combined capacity of the lot of them?

    They won’t be charged in parallel, but what is the implication of this? To me it feels like this *could* be bad?

    I fully accept that they would ideally be both at full voltage before connecting…

    • Hi Chris
      The internal forward resistance of a discharged battery is quite low, so connecting a second battery in parallel that has even only a small voltage difference will result in the batteries trying to equalise quickly, hence the fuse will blow. Ideally the best way to equalise batteries is to connect them to a smart charger individually and let the smart charger do it’s magic and once all the batteries have been through this process, only then attempt to connect them as a bank. Personally I use a C-TEK multi stage smart charger and haven’t had any issues using this method. Once the batteries are connected in a bank, I use the C-TEK again to do one final charging session and from then on use the C-TEK to equalise the bank on a periodic bases even though the bank is discharged/charged by the caravan services. Good batteries are expensive and I try to look after them as best I can. So far my batteries are 5 years old and not (touch wood) showing any signs of ageing.

      With regard to connecting batteries of differing Ah ratings, the smaller Ah battery will usually fair less well going through charge/discharge cycles when connected in a bank.

  13. Simon, what is the ” smart charger ” you mention? Does an ordinary charger not work on lesiure batteries?
    Ta, John Cole

  14. Hi. All thing being equal, and everything wired as you suggest in a Part 1 and Part 2, where do I connect me charger? Thanks for info, it makes good sense, however do I connect charger + and – to separate battery terminals, and what if I have three batteries?

    • Hi
      Your charger should be connected to the `=ve terminal where you take the feed from the battery pack and the -ve to where the main -ve connection to the battery pack is.
      The charger should always be connected to the main connection to and from the battery pack no matter how many batteries there are in the pack.

  15. cuillinridge said:

    Hi Simon, not sure if you’re still responding to comments in response to this really useful and informative post. I have wired two batteries in parallel as you’ve shown. The positive from battery A runs to the blade fusebox for the appliances and I have the earth/negative coming back from the blade fuse box. My question is – does is matter if I connect this earth/negative from the appliance fusebox to the -ve of Battery A, or should it be connected to the -ve of Battery B? Thanks

    • Hi
      If the +ve is coming from battery A, the -ve should be connected to the -ve post on battery B. This will eliminate any voltage drop losses due to cabling between the batteries for both charging and discharging.

  16. Robert Gilbert said:

    Hi..I have a new motorhome with a 100watt solar panel that is permanently connected to the leisure battery. However there is no provision for charging the Vehicle battery and I am thinking of the time when it is parked up (including winter storage) with the alarm set. From what I read this can run down the VB in a coup[le of weeks.
    have I fitted a 12 socket, fused at 10 amp, near the VB (which is inside) and have made a lead (as suggested on an internet forum ) which can connect the LB and the VB in parallel but I have included a shottky diode (VF=.0.3 volts) in line, in the belief that solar controller cannot “see” the VB and will (my theory) will continue to keep the LB fully charged but also trickle charge the VB at the same time. I have not simply connected them directly in parallel as they are different types of battery. Is this a madcap scheme !?

    • Hi Robert
      Sorry for the delay in replying… I’ve been mulling this over.
      I can’t see anything wrong with what you have done. As long as the solar charger output voltage is above the voltage of the LB and VB, then the VB will charge. The only thing I would caution about is forgetting to unplug the lead before you start the engine just in case it has any detrimental affect on the vehicle to leisure battery charing system.

      For the future, you might want to look into a MPPT solar charger with dual output. These are designed to do exactly what you are trying to do but being MPPT you will maximise the potential of your panel.

  17. Alex maynard said:

    Hey mate, really loving your write ups! Very intuitive. You made mention of doing a post about wiring two batteries in parallel to power a winch, I can’t seem to find that post or is it not done yet?
    Look forward to hearing from you.
    Alex.

    • Hi Alex
      I changed my tow vehicle last year and I’m not sure if I will be installing a winch on the new one.

      • Ah that is fair enough. Cheers fpr your prompt response. Would placing a winch in the circuit like you have shown in this post do anything majorly wrong?
        Cheers

      • Hi Alex
        The main issue is protecting your starter battery, I’d go for running the winch off a second battery (or pair) that is not connected to the starter battery and invest in a DC to DC charger so that when the engine is running once the starter battery is topped off, the winch battery gets recharged.
        A DC to DC charger is not the same as a split charge system, as a split char system will only put the alternator output into your battery. A DC to DC charger has a smart multi stage charger that will take the battery through bulk, absorption, float and DS stages to make sure you get the best out of it.

        The other thing is it will also protect your alternator so you can leave your engine running while operating the winch without fear of causing damage to it.

  18. Hi there, Great Read. I am adding a new 29 group battery underneath to the frame in the back of my van to increase my power reserve. It is much larger than my starting battery. I have a 130 amp alternator. I will be using an 80amp continuous duty/150 amp surge isolator solenoid for charging. (Assuming everything is wired and fused correctly,) When the engine is off and the “house” Battery gets drained. When I go to start the engine there will be an equalization and i am wondering how the voltage regulator in the alternator is going to respond to this process as well as how it will respond to the to different types of batteries when charging, Thank you for your thoughts.

    • Hi John
      The solenoid should not operate until the alternator is providing an output, therefore there should not be an issue with the batteries trying to equalise. The alternator will just provide a charging voltage for the type of battery that was specified for the vehicle, if you have a different chemistry house battery (AGMI, AGMII, GEL,LiPo etc) it would be better to install a DC to DC charger that can accommodate the house battery chemistry type.

  19. Thanks for really helpful info. Previous owner had connected two batteries wrongly, which I’ve now corrected as per your diagrams. Which terminals should I connect a Ctek smart charger to: + & – on one battery or + on one battery and – on the other?

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