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What is this cell trying to tell me?

RipK

New Member
Joined
Oct 11, 2022
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10
100 AH battery was continually tripping off on COV before fully charged. I tried charge and discharge within the balance parameters, I did this 12 times and only saw 6 mv improvement in balance (DV was approx. 0.320 v). After researching this forum I mustered confidence to top balance the cells individually while still connected to each other. Using a 10 A bench power supply and starting with the cells charged as full as the BMS would allow I balanced each cell. As expected it took quite a while and three cells balanced fine and at finishing stage the time for the current to drop from 200 mv to 50 mv was approx. 5-10 minutes. However, the other cell took much longer to balance and the drop from 200 mv to 50 mv took 30 min! I then discharged the battery to 80% and then charged it and it fully charged, the DV was 12 mv, I thought great it worked ! But then as it reduce to resting voltage the problem cell voltage dropped three times quicker than the others and DV grew to 45 mv. A further discharge to 80% and recharge produced C MOS cutoff on COV and the DV increased to 0.245 mv!

I tried to get my head around this but failed, it seems this cell is slow to charge (takes more current per mv increase) but is also fast to discharge. Is this a bad cell, if so how do I quantify it to claim under warrantee? Any explanation would be welcome.
 
100 AH battery was continually tripping off on COV before fully charged. I tried charge and discharge within the balance parameters, I did this 12 times and only saw 6 mv improvement in balance (DV was approx. 0.320 v). After researching this forum I mustered confidence to top balance the cells individually while still connected to each other. Using a 10 A bench power supply and starting with the cells charged as full as the BMS would allow I balanced each cell. As expected it took quite a while and three cells balanced fine and at finishing stage the time for the current to drop from 200 mv to 50 mv was approx. 5-10 minutes. However, the other cell took much longer to balance and the drop from 200 mv to 50 mv took 30 min! I then discharged the battery to 80% and then charged it and it fully charged, the DV was 12 mv, I thought great it worked ! But then as it reduce to resting voltage the problem cell voltage dropped three times quicker than the others and DV grew to 45 mv. A further discharge to 80% and recharge produced C MOS cutoff on COV and the DV increased to 0.245 mv!

I tried to get my head around this but failed, it seems this cell is slow to charge (takes more current per mv increase) but is also fast to discharge. Is this a bad cell, if so how do I quantify it to claim under warrantee? Any explanation would be welcome.
Sounds like one cell has a much higher self discharge rate.
Probably means you need a new cell.
 
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