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Old Oct 24, 2006, 01:28 PM
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USA, FL, Apopka
Joined Dec 2000
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A123 potential problem with building packs

I finished my adapter to allow balancing my 4S A123 packs with my Astro Blinky. It works fine..... but as I now have access to individual cell voltage I have a concern.

It is my belief that DeWalt and A123 are NOT matching and balancing the individual A123 cells before assembly into a DeWalt battery pack. I believe they rely on the internal balancer to bring the cells into alignment on the initial charge.

What does that mean? The individual cells in a DeWalt pack may or may not not be matched in voltage or charge when you disassemble the pack to build a RC pack.

On two of my five 4S packs made from two DeWalt packs I found a voltage difference in excess of 0.4v. The other three were all within 0.1v.

On one pack I had to individually add over 400mah of charge to bring it into alignment. Luckilly I have not flown that pack to over 60% of capacity or I might have driven that cell into a serious overdischarge.

My advise is to check the voltage and charge state of each individual cell before assembling into a finished pack.

Hope this saves somebody from killing a pack.

Brad
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Old Oct 24, 2006, 01:42 PM
Registered User
Slovakia
Joined Apr 2005
3,386 Posts
Thank You for pointing this up.
Now, how deep is too deep for A123?
Of course, cell reversal is for sure too much, so, zero could be the bottom to start with..
In theory, A123Racing says that down to 2.0V is normal, and even 1.0V is still not harmfull - but it is probably voltage under load.
Easiest check of the cells during flight could be done at approximately 1.1V.
I hoppe that someone could post more about real experiences, and data with graphs of discharge voltage effect - but there are two voltages to take in mind, rest voltage as well as minimin under-load voltage.
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Old Oct 24, 2006, 03:18 PM
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Haralson County GA. USA
Joined Oct 2004
34,839 Posts
I had a cell go to 0.02 volts in a 4 cell pack after it had been cycled anfd balance had been checked. It was a low rate discharge to 8 volts.

http://www.rcgroups.com/forums/showthread.php?t=585291

I also have a cell that was 0.4V behind the other 3 durning the first charge and had to charge it seperately to get it balanced to the others.

Durning a normal(?) med. to hgh level diacharge of 1800 to 2000 mAh the cells seem to stay balanced very well. If I were going to use at low discharge rate levels or to near max. capacity I would use a LVC which monitors each cells voltage.


Charlesi
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