How to: Connect two batteries in parallel


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 bodywork. This will stop the chance of Eddy currents in bodywork.

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.

S

PS

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

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Copyright © 2011 – 2014 Simon P Barlow – All rights reserved

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

  1. 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

    Colin

  2. 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

  3. Malcolm Dockerty said:

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

  4. 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

    John

    • 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.

        S

  5. 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
    John

    • 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.

      S

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

    regards John

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

  8. 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.

      Cheers

      Simon

  9. 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:

      t=H(C/IH)k

      (k is a power)

      where:
      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

      S

  10. 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?
    Thanks
    Tim

    • 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

      Simon

  11. Tim Cooper said:

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

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

  13. 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.
    Thanks

    • 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.
        Thanks

      • 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 http://www.sterling-power.com/ or search for “alternator to battery split chargers”.

        S

      • Many thanks

  14. 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… http://www.selectsolar.co.uk/cat/165/choosing-a-solar-panel

      Cheers

      S

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

    • 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)

      S

      • 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.

        S

  16. 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
    Alan

    • 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.

      S

  17. 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.

  18. 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.

      S

      • 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.

        S

      • 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.

        S

  19. 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,
    Neil

    • 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

      S

      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.

        S

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