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”


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.



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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20 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


  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.

  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


    • 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.


  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

    • 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.


  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.



  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:


      (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


  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?

    • 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


  11. Tim Cooper said:

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

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