Caracteristic of cell that lost capacity

I need your opinion about that.

I am working on many used dewalt pack and I try to detect easily and isolate bad capacity cells in the pack.

What are your best way to do that?

-Monitoring voltage on cell that drop prematurely?
Normally, it seems pretty simple to fully charge and try to balance all the pack and after few minutes, to discharge them without balancing and to monitor each cell voltage. the cell with voltage that drop first would have less capacity then the last.

-Measuring cell impedense in miliohm using specific instrument?
using my charger, i measured impedense on individual cells and some are at 16miliohm, some are at 20miliohm.. Does the one at 20 have absolutly lower capacity? I assume that the higher is the capacity the lower the impedense right?

-monitoring cells that reach their max charging voltage first?
I observed that when a cell is charged individually, some of those that are bad cells would charged faster than other... taking less mAh...<


All battery pack are like christmas lightbulb cord, when they are in serie, if one bulb blow, the rest would stop functioning... the reason why we need to keep our pack completly balanced...

Doc

I've spent hours probing individual cells on my packs....

Process i usually do:

1- Fully charge
2- Take note of full pack voltage
3- Drain pack using X load ( i use 200w lightbulbs in paralell using dual bulb fixtures on a piece of 2x4 , more bulbs.. bigger load .. )

4- after X amount of time ( depending on nimh/nicad/lithium/etc.. ) and load applied i check each cell in the pack one at a time during discharge.

The first cell to drop below spec determines the packs capacity. :wink:

Nimh/Nicad = 1.0v per cell under load
LiMn = 3.0v per cell
SLA = 10v per brick = 30v on a 36v pack

When testing Nimh/Nicad i cycle the pack at least 3 times with a shallow discharge ( 50 % capacity max ) then re-charge and trickle for a few hours... all on consecutive days.. if you allow a week to go by.. re-cycle again as the packs are already out of wack ( my NExcell packs anyways.. )

With Li it's not so important to cycle them to ballance out.. as the voltage of a cell tells alot about state of charge.. ( not so much with LiFE... from Oatnet's tests.. hmmmm )

Testing batteries can be a religious experience.