10s4p.. 1 parallel group unbalanced.. thoughts?

So i built my 10s4p battery about 6-8 weeks ago and its been running great till recently. I never did a full range test at the beginning because with all those amp hours I have plenty of range to ride hard without it dying before i got home. At any rate I did a couple longer and harder rides recently and the bms was cutting off power at 36v …also started to notice my battery meter was only showing 95% full charge or 40ish volts. I thought it was a bms issue.

So today i did a hard ride and the bms cut me off after about 6miles hard riding… did the push of shame home and ripped it open… so

cell group 1 was reading 2.9v all the others were about 3.8 - 4v so the good news is it appears not to be a bms issue and the bms was doing its job (good boy) AND my spot welds were a pain in the ass to rip off so that means theres no issues there… no other visible signs of anything wrong.

all batteries were brand new when assembled (PF cells)

I am currently balancing the 4 cells (individually) back up with my b6

but any insight into what may have happened? thx

My guess would be a bad cell and bad luck. If one of the cells is faulty then the imbalance during charging would slowly get worse until all the cells in that group fail. The BMS is only designed to cope with less than 100mA in balancing so would not be able to compensate for the effects of that defective cell. This is also a good reason for having all your cells fully charged for the build so the BMS has less balancing to do.

well see… all were new upon assembly at 3.7 and charged up fine… just balanced each cell back up… re welded all and giving the pack a charge up… will let it set a day and see what happens…if no drift while sitting will lightly re-package it and take for a few rides and see what happens!

Good luck on the retest.

Let’s hope they are okay. Remember if cell group one was resting at 2.9 imagine what it went down to under load.

…or perhaps the 2.9 is what triggered the bms in thw first place… Maybe we’ll never know … But Will see in the am

Hope everythings okay. I have once had this issue but it was with lipo batteries. I made a 10S1P with single lipo batteries and because of the high current 1 battery became very hot and the voltage was much lower than the rest, so it was maybe the ‘weaker’ cell so it will fail first? Maybe you were pulling to many amps? What kind of batteries you use, 30Q?

re-hash update on this just for fun… this battery has been sitting probably since sept since i built my 30Q… but now that i have enough parts for a dedicated AT board and a street board I was going to bump this basically new battery up to 12s… so i bought a few more batteries and tore it apart the other day. all groups were perfectly in balance except the one in question.

all batteries in group 1 were at 1v =( oh well guess that proves it… anyways, replacing that next week as well as the added to groups two bump up to 12s and then we ride again…!

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