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Cells voltage goes down while charging..

tommix

New Member
Joined
Aug 22, 2022
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7
Location
Lithuania
Hi maybe this is stupid question, recently bought eve280ah v3 cells. 24v system. Attached to cell 5 and 6 wire to feed my esp32. But when those 2 cells once hit bottom and bms cut the output then i just bought usb pcb and now feed it with first ant last cell (whole pack feeds energy to usb). And when first charged those 2 cells were lower than others. All seems logic, those were depleted the most.. ok.. Then had to use battery and discharged it bellow the level those 2 lower voltage cells were and all cells became equal (~ 3.31v, delta was about 0.003v). Than when cheap electricity was available i started to charge them. All looked nice, balanced..at least till 3.41v and then suddenly those 2 cells started to drop voltage while others get higher voltage. See picture it shows 1 cells that was lower and other that was like rest of the 6 cells.. HOW this is possible??
How can voltage drop while charging? Is this because of other 6 cells have much lower capacity? Why it was ok till 3.41v and sudden started to drop.
This is with 2 cells as i mentioned, but picture only 1 low and 1 'normal' to make it cleaner.

PS. also when stopped charging (6 cells were at 3.5v and current about 2A.. - used voltage limit @28v so amps dropped) and started to use battery, now the lowest cells still are higher voltage than those who seemed to be more charged. Is this just unbalanced cells problems (both x-files)?

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Those are the two highest voltage cells, the balancer may be kicking in to drain them back a bit to let the other cells catch up?
 
First and foremost, stop connecting persistent loads to individual cells, no matter how small. That will throw off the BMS in ways you won't see until it's too late, because not only is it draining one cell differently from the rest of the pack, but it's introducing a change in impedance that the BMS can detect but doesn't know how to manage. You can sometimes charge one cell at a time to address balance issues, but not two cells at the same time, because the BMS will get confused and do strange things. Also, when attaching to an individual cell, the power supply you use must be electrically isolated from the rest of the battery, or else it will cause dangerous voltages on your BMS balancing sensors. The battery cannot be in use or even connected to a system while doing anything to an individual cell unless you can be 100% certain everything is electrically isolated.

then i just bought usb pcb and now feed it with first ant last cell (whole pack feeds energy to usb)
What does the USB PCB do? Why would you attach it directly to individual cells AND the whole pack at the same time? This in itself looks like at least part of the problem, if not all of it.

Given what I have gathered from your description, I think you're causing the problem yourself. Disconnect everything from the individual cells and charge completely. Leave it at full charge for a few hours to allow the BMS to balance the pack, and then review the overall condition of things.
 

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