# Batteries in parallel for higher mAh

Discussion in 'Power Electronics' started by dbyrd26, Nov 13, 2014.

1. ### dbyrd26

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Sep 16, 2013
I know that when you connect 2 identical batteries in parallel you get the same voltage but twice the capacity. So, my questions are 1. What happens if you connect 2 batteries that are not the same voltage? And 2. Should one use a diode when connecting batteries in parallel?

2. ### Gryd3

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Jun 25, 2014
If your batteries are not at the same voltage, then the higher voltage battery will discharge into the lower voltage battery.
If they are rechargeable batteries, then this will balance itself out, just so long as the difference is not so great that one damages the other. It's always a good idea to make sure they are at the same level prior to connecting them.

A diode will prevent one from feeding into the other, but has a draw-back. It will not allow the batteries to be charged in-circuit, and they cause a voltage drop... which could result in up to 0.7V to be lost, and if the battery voltage is low to begin with, this will cripple your power supply. (ie... two AAs in parallel, with a diode on each will only give you about 0.8-0.9V even though the batteries are 1.5V)

And, when connecting them in parallel, the 'Current' is additive, the voltage stays the same, so you would get the sum of both batteries mAh, but you would also have a new 'battery' that is capable to putting out more Amps at a time. This is why some Diesel trucks use 2 batteries in parallel, so they can push well over 100A into the starter to get the engine moving.

Oh... if you do plan to use two different batteries, that have a different voltage, then a diode will, like stated above protect one from back-feeding into the other. This will essentially allow the lower voltage battery to be a 'back-up' for when the higher voltage battery finally dies enough that it's voltage drops down.

3. ### dbyrd26

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Sep 16, 2013
Ok. This is very helpful thank you. I have 2 lipo batteries, 1 is 7.2 volts and the other is 3.7 would this cause issues for me?

4. ### Gryd3

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Jun 25, 2014
Yes.
The 3.7V lipo would not be used until the 7.2V was below half it's voltage, which would not be very good at all for the 7.2V battery.

5. ### dbyrd26

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Sep 16, 2013
Got ya. Thank you!

6. ### BobK

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Jan 5, 2010
If you really need to add the capacity of these two batteries together, you would need to put them in series and then use a DC to DC converter to bring the voltage down the desired level.

Bob

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7. ### dbyrd26

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Sep 16, 2013
I didn't think that capacity increased in series

8. ### davennModerator

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Sep 5, 2009
That's right .... so you would just need a higher capacity battery

its a really bad idea to parallel uneven voltages, it can cause the lower voltage battery to cook and maybe burst into flames

Arouse1973 likes this.
9. ### Anon_LG

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Jun 24, 2014
Agreed, Lipo batteries are EXTREMELY dangerous if misused, I could tell you a story (true story) but it might put you off.

10. ### dbyrd26

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Sep 16, 2013
Well, I'm not going to connect these in parallel haha. I've decided against it, and I have seen some pretty crazy stuff happen with lipo batteries. I'm working on a project right now and am trying to make a decision on what batteries to use... I have 2 8650 cells but they were quite a bit larger than I expected. I also have a single flat lipo that I'm thinking about trying. But the reason I asked about these 2 is because I happened to come across them at work and thought the size would be good.

11. ### Kiwi

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Jan 28, 2013
"This is why some Diesel trucks use 2 batteries in parallel, so they can push well over 100A into the starter to get the engine moving."

You are a bit light on the batteries fitted to some large American diesel powered 12v trucks. They can have up to four 1000CCA batteries fitted in parallel, giving a possible 4000A.

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12. ### Gryd3

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Jun 25, 2014
With Bob's trick it does...
The DC-DC buck converter will output the same power that is put into it.

So a 7.2V lipo rated at 2400mAh, can output 3.7V at a little under 4800mAh.
Power = Voltage * Current

(Unless I fubar'd something here.)

You will need keep on eye on the voltage level of the batteries independently if you simply hook them up in series and go. The lower capacity battery will die first.

13. ### BobK

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Jan 5, 2010
It does if you then reduce the voltage via a DC to DC converter.

Say you put 3 nominally 3.7V LiIon batteries with 1000mAH capacity in series.

If you then run them through a buck converter and get 3.7V at 1000mA, the batteries will last 3 hours because they will only be draining (at a rate of 1A) for 1/3 of the time. This is how a buck converter works. It switches the power on and off to keep the current flowing through an inductor. If you reduce the voltage to 1/3 they only need to be on 1/3 of the time in order to keep up that current. Actually it is a little more than that. Typically they are only 80 to 90% efficient.

Bob

14. ### Kiwi

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Jan 28, 2013
Bob, instead of using three batteries in series with a buck converter, would it not be better to use three identical batteries in parallel?

15. ### Gryd3

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Jun 25, 2014
It would be yes,
But Bob's method above was a 'last-resort' for using the two specific batteries the OP currently had which were rated at 7.2v and 3.6V.

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Dec 18, 2013
Yes absolutely correct, DO NOT DO THIS.