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Old Sep 08, 2006, 11:25 AM
crashdummy6's Avatar
Joined Dec 2005
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Question on P connecting lipos

Two batteries, same voltage and cell count (3S) but differing capacities, say 700 and 800mah. Can they be connected with an adapter into a 3S2P without worries of different capacity, or does the capacity have to match as well. I am guessing they have to match but am hoping I am wrong.
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Old Sep 08, 2006, 11:42 AM
Southern Pride
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Haralson County GA. USA
Joined Oct 2004
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This will start another great debate but no they do not have to be the same capacity.
They need to be very close to the same voltage and it is best to connect them in paralel while they are mostly discharged and charge them in parallel. It is also best if they have approximatley the same C rating and past history.


Charles
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Old Sep 08, 2006, 01:16 PM
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My experiments and experience supports Charles'.

After you've gained some experience, you may find you can be a little less conservative, but I would advise you to get the experience first.

Your example of 700 and 800-sized packs is fortuitous. The smaller the packs, the easier it is to contain collateral damage if there's an accident.

- RD
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Old Sep 08, 2006, 03:29 PM
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Joined Dec 2005
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I will be charging them separately with a balancing charger so the voltages should be fine. I was just wondering how the depletion of the stored energy would balance out. If current flow is the same in all parts of a circuit, I was thinking the lesser capacity battery would drain low well before the larger capacity battery causing its cells to drop in voltage sooner. I am probably missing a basic principle somewhere. It has been a while.
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Old Sep 08, 2006, 03:39 PM
Deletedfor proving Nauga wrong
Joined Mar 2005
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I find it easier to hit the same voltage by charging the two packs. Since the LiPo charger automaticly stops at the same voltage every time... (+/- less than .02 v)

I then connect - to - and put a resistor between + leads... and verify my self-ranging DVM indicates 0V across the resistor. (that indicates less than 1 milivolt differential...)

If you take the step of doing the differential voltage check across a resistor.. you won't have a problem paralleling packs of the same cell count. (3S, 4S... Parallel count doesn't matter)

Once they are paralleled... if thats your primary use of the packs... keep them paralleled. one charge ballance pack A... next charge ballance pack B... or parallel the taps plugs too (using the resistor trick) and you can ballance both packs at once.

*************

Smaller pack may indicate a lower voltage under load than the larger pack... but that will reduce the current draw on the smaller pack... You'll find that as soon as the load is off, both packs have the same voltage.

Internal resistance of the cells will take care of keeping one pack from draining before the other. (also with them paralleled... the high pack would charge the low pack until they hit the same voltage. This is why parallelling cells works)
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Old Sep 08, 2006, 03:41 PM
Boffin
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Victoria, BC, Canada
Joined Apr 2001
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Quote:
Originally Posted by crashdummy6
I was thinking the lesser capacity battery would drain low well before the larger capacity battery causing its cells to drop in voltage sooner. I am probably missing a basic principle somewhere. It has been a while.
If they are in parallel, they will always have equal voltage. The "weaker" battery will supply less current.
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Old Sep 08, 2006, 03:57 PM
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The current flow is only the same in all parts of a SERIES circuit. It's voltage that is always the same in a PARALLEL circuit. The current in each part of the circuit depends on the effective resistance of each side. In general there's nothing stopping you have ing 2A flowing in one side of the circuit, 1A in the other side and you will then have 3A once they have joined together.

Since the voltage is always constant on the 2 batteries they basically balance the load between them. One cannot drop the voltage sooner so it sort of compensates by reducing its current output to suit the voltage that it's held at. The most powerful one will contribute more current than the weaker one. So provided they're reasonably well matched batteries (particularly C ratings) they will both end up exhausted at approximately the same time.

If you had something like a 700mAh 20C battery and an 800mAh 6C battery things would get more interesting .

Steve
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