Recently George asked in the comments on our article about connecting batteries in series and parallel at the same time how he could make 36 volts from 12 volt batteries, while at the same time make the capacity (amp hours) add together so that each 12 Ah cell would combine to make at 36 Ah total battery.  Here is what he wrote:

I have three 12V12Ah batteries. I need 36V36Ah.Is the following thought correct ?

I connect all the plus together, then all the minus together. The result is: in parallel I would have 12V and 36Ah.

Then I connect from first battery plus to second battery minus and from the second plus to the third battery minus. This would be series and I achieve the 36V.

Thanks for your help Robin.

And the answer is no, it doesn’t work quite like that.   In isolation George is correct.  If all he wanted to do is make 36 Ah at 12V then his first idea would work.  Three 12 volt batteries in parallel makes 12V @ 36 Ah.  And then if he took those three batteries, disconnected them, and reconnected them in series like in his second idea, he would have 36V, but only at 12 Ah.

But if you need to do both at the same time, it gets a little more complex.  Take a look at the article and study the diagram.    In the example we needed 24 Volts and 200 Ah.  We had four 12 Volt batteries, each with 100 Ah.  We took two batteries in parallel to make one big battery with 12 V @ 200 Ah, did it again to make another big battery with 12 V @ 200 Ah, then took those two batteries and put them together in series to make 24 Volts at 200 Ah.

George, what you need to do is get (brace yourself) … NINE batteries.  Take three of them, put them together in parallel to make one big battery with 12 Volts and 36 Ah, then do it twice more to make 2 more BIG batteries at 12 Volts and 36 Ah.  Take all three of your big batteries, and put them together in series to make a HUGE 36 volt battery, with 36 Ah.  Like in the last article, I’ve drawn a simplified diagram that shows how to do it.

So this might seem a bit much, especially since you’re only working with 36Ah in total.  You’re probably better off just buying three 12 volt batteries with the minimum capacity you need – you could also look into getting a 36 volt cell as well.  Either way, the concept above stands.  I hope this is useful to you.