# Batteries - Switch between series and Parallel

Discussion in 'Power Electronics' started by Lord_grezington, Aug 17, 2013.

1. ### Lord_grezington

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May 3, 2013
Hello Again everyone

I have been looking at this for a couple of hours now, so decided to post it on here to see what responses I get.

I am looking to get 36Vdc from 3 x 12Vdc batteries (Lead acid at the moment, however needs to work with lithium in future). I will need a max current of around 15A in both series and parallel configurations.

I am looking to use a switch (probably one of these with the centre off position http://www.farnell.com/datasheets/579005.pdf).

It is very simple to switch between series and parallel when only 2 batteries are involved (24V), but I cant seem to find a simple solution for 3 batteries 36V.

I keep thinking its possible and i'm just getting a "stupid moment".

Can anyone help?

2. ### john monks

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Mar 9, 2012
You need a 4pdt, four pole double through, switch and the you can easily connect three cells all in series or parallel. Just make sure that the switches are break before make and I believe most power switches are.
I am sitting in my truck with my iPad and therefore am unable to draw you a schematic but if you draw all three cells side by side it is easy to see how to connect the switch.

I think that to analyze this problem is to lay out the three cells and ask yourself, "How do I switch all three batteries in parallel" and then "How do I connected all three batteries in series?" first ignore the kind of switch you need, draw out your circuit, then later on choose your switch. You have three different 8pdt switches you can choose from. All should work.

Last edited: Aug 17, 2013
3. ### Lord_grezington

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May 3, 2013
Hi John

I have tried this, I cant seem to get it right. As I say it sounds easy but I cant seem to get it to work (even with the 4pdt).

4. ### john monks

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Mar 9, 2012
Let's take this a step at a time. Can you do this with a whole bunch of single pole switches? If you can the your halfway there.
Just for the time being I'm forgetting about the lithium batteries.

5. ### Lord_grezington

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May 3, 2013
Unfortunately not, it needs to be done with just one switch.

6. ### john monks

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Mar 9, 2012
I was only suggesting that you logically on paper do this arrangement and the finding the appropriate ganged switch to use. I see three on the datasheet you provided that should work.
I'm assuming that you want to switch from three 12 volt batteries in series to three 12 volt batteries in parallel. Am I mistaken?

Last edited: Aug 17, 2013
7. ### Lord_grezington

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May 3, 2013
No, you are not mistaken.

I think I am having a moment (its late night here) that I cant get it done.

can you draw the schematic on paper, take a picture on it with you Ipad then attach it so I can see you idea?

8. ### john monks

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Mar 9, 2012
I will be happy to do that with my other iPad but I might need somebody to help me do that. Or maybe I can just email you a picture. This is a plea for help.

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May 3, 2013

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May 3, 2013

11. ### john monks

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Mar 9, 2012
Sorry about dropping off but here goes.
You have three 12 volt batteries, B1, B2, and B3. Now you want to connect them in parallel with a switch with at least four sections, S1, *S2, S3, and S4. So you have the common of S1 connected to the plus side of B2 and the first position connected to the plus side of B1 and the second side connected to the negative side of B1. Now you connect the common of S2 to the plus side of B3 and the first position to the positive side to B2 and the second position to the negative side to B2. Now you connect the common of S3 to The negative side to B2 and the first position to the negative side to B1 and the second position connected to nothing. Now you connect the common of S4 to the negative side of B3 and the first position to the negative side of B2 and the second position connected to nothing.
The power is taken from this system from the positive side of B1 and the negative side of B3. Now you are done.
So the first position is for 12 volts and the second position is for 36 volts.

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Try this. It's just for 2 batteries, but you do the same thing between each pair of batteries, so just extend it for 3 batteries.

Does that help?

edit: Safety tip... Connect only the wires required for the series connection first. Ensure you have 36V in one direction and 0V in the other. Then connect the other wires *carefully*. I would advise you to have the switches in the 12V position and do a quick spark test before connecting the wires to make sure you don't short a battery out. Even better, connect a 12V bulb first, and only try the wire if the bulb doesn't light up.

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Last edited: Aug 25, 2013
13. ### Lord_grezington

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May 3, 2013
I still cant seem to get it to work without a short with 3 batteries. Above you use one common for a positive, the other common for a negative - if a third battery is introduced, is it the positive or negative on the common? I can get it to work ok in series but it then shorts in parallel.

14. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
The meads from the switches go all the way to the ultimate +ve and -ve ends.

I suspect that's where you're making the mistake.

Just consider where you want the batteries connected in series (easy - you've got that) and then in parallel (each +ve goes to the same place and each -ve goes to the same place).

If you still can't figure it out (or if I find I have a few moments to spare) I'll draw it up for you.

15. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
I think this is what they mean.

16. ### Lord_grezington

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May 3, 2013
Thank you - seems obvious now.

17. ### Yaser43082

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Jul 20, 2013
If u need 36v DC, I would create a DC-DC Convertor instead. It is a little more work, but if designed right, can solve your problems

18. ### Lord_grezington

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May 3, 2013
Its a bit late on this design, but I will use the boost converter the next time round. not sure what the cost of the inductor is likely to be as I need around 15A. I will look into it in a bit more detail when it comes up again.