What charge state best to check bad cell?

[Swe]

I have a battery where one paralell pack (8P) gets drained faster. And it charges faster too, though last time (if I remember correct) it charged slower so I am not sure anymore... 0,1V or so in difference. Anyway when is best time to take battery apart to compare the 8 cells? I was thinking maybe 1 of the cells differ from the other 7, or maybe a bad assembly.

Is it best to take apart after a longer ride and the cells differ from the other, and then take cells out before they balance between each other? And just compare their voltage. Or is this method not possible anyway? It is a no solder-pack so it is not too hard to open up.

 

[dogman dan]

Well, the best time to short out the pack, and fry your fingers, is when its at the lowest state of charge. None of us seem to do this though, we always KFF when re assembling shit, usually at full charge because we balance the cells before we put them back together.

I'd say just anything except full would be good, just discharge it some. once apart, you have to charge and discharge each cell individually, watching what happens during both with a voltmeter, to find that weak cell causing the problem.

But lets back up a bit. One group of cells is .1v worse than the other. Id call that normal for any assembly of a large number of cells. As long as it hold at that point of out of balance, why is this a problem? Do watch it, and see if it gets worse. But really, no such thing as a perfect battery pack.

 

[Swe]

But the cells are almost new (or at least not used before) so I was thinking they should match, and the Adaptto just show it so clear...

I dont have anything to charge/discharge cells that also measure stuff. Maybe if I spread all the cells of the bad block to all other P-blocks (change with each other) then the fault should move to another block? (If the fault is one bad cell). Can I mount it together when there is 0,1V difference on one cell? Probably the one cell will have lower voltage compared to the other 7.

 

[dogman dan]

I'd just keep an eye on it for now, if you have no way to test the cells individually.

look up testing internal resistance on y tube, you will need a way to discharge a cell or string of cells, and a voltmeter to do an IR test. One cell or p string, can be put under a load with a simple 12v turn signal light bulb.

A very handy tool for balancing a paralleled section is a small 50w RC charger. with it you can charge up the low voltage section, then watch it to see if this ending up low is typical.

With a new set of cells, its entirely possible it is slow to charge to full because it has more capacity than the other strings, not less.

Balance er up, and then see what it does in the future.