LiFePO4 pack rebuild - what are my options?

[oddjones]

OK - just gave a single cell a bit more manly a test - I hooked it up to a car cigarette lighter - I'm not sure of the actual draw of this, but WikiPedia seems to think about 10a at 12v = 120W. The voltage sank to 3.19v immediately and the cell gave a constant 1.5-1.6A for 50 minutes before the voltage started to drop towards 3v and current tailed off to about 1.35A.

I stopped the test there as I didn't want to damage the cell. Once off load, the cell quickly recovered to 3.17v

The spec sheet for the cell (https://na4.salesforce.com/sfc/play/index.jsp?oid=00D6000000076DF&d=05D600000004CLYEA2) states a 1.4Ah capacity and a max continuous discharge of 2.7A

I reckon these cells are just fine. They are (cinema voice) "Hard to Kill"

 

[dogman dan]

yeah, sounds like some capacity is still there.

 

[oddjones]

Well, Dogman - it looks like you were right!

I rebuilt the pack, using spring connectors, bus bars and plywood. I recharged every single cell using a 3.3v Camera charger, put it back together, connected it in serial and WOW!

I got the 72v buzz I'd been looking for - for about a mile - then my serial pack turned from 80V into 16.3V

?!!

I had my meter on me of course - connected the terminals to the "new" battery and got -23V - that's minus 23 volts... How in God's name does that work?

I connected it back up and got 76v again, but the same thing happened after a couple of minutes - My question is - can a single bad cell cause this kind of problem?


I rigorously recharged every single cell and only got less than 3.3v with one cell - I connected it up in a string of 7 hoping that the others would "pick it up" - I guess that was a dumb thing to do?

That said, if I test across all my 12 strings I get 3.3v across them all until I put it under load, then I get this wierd "reversing" effect.

Needless to say I've taken the battery out of serial with the good pack, which doesn't seem to be affected in any way...

Obviously I can't rebuild the pack without the cell as I'd then have too few cells for a workable pack - although the pack is obviously pretty useless to me as it stands anyway. Good for a torch then?