I have a 4200 race pack that I think has a few blown cells and I would like to replace each bad cell. How do I tell which ones are good or bad? Thanks, Doug

voltmeter

So what do you do look for with the voltmeter?Do you have to disassemble the pack and see how many volts each cell has?Ive only got 3 packs that are 3800 4cell packs for 12th,they charge up a little above 6 volts,thats where they should be right?I dont think anythings wrong with my packs i am just interested in the answer to Oxymorons question.

Usually if cells going bad, it will get noticeably hotter than surrounding cells when you charge it. The bad cell will also get hotter when you run it for a few minutes. If you run the batt all the way down all the cells will be hot and won't be able to tell them apart. Run it long enough for cells to start heating up and then check pac for a hotter cell. That's usually the bad cell.

If the pac is going south and you're serious about repair, you're going to have to pull it apart anyway and voltmeter will ID bad cell easily. I've done this with several pacs and rebuild them in side-by-side config so I can use a disch tray on them as part of charge routine.

When charged, cells should be close in voltage, thats what you're looking for with voltmeter. If 3 cells are say 1.4 - 1.45v and 3rd cell is 0.78v, you've just found the bad one. Cell internal resistance goes up when they start going bad and the extra resistance causes heat and also is why voltage is lower.