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single cell show higher voltage when charging, and lower voltage when discharging.

colevr

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Joined
Feb 1, 2022
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1
Hello everyone.
I searched a lot in various forums, but it seems I wasn't able to find a situation similar to mine.
I have a system with 4 lifepo4 cells, 280 AH, bought on AliExpress 1 year ago. I have a PEYO BMS which is a DALY clone.
I updated the firmware to version 210629.
Initially to balance the cells, as I don't have a suitable home charger, I left them connected in parallel for various weeks.
I also installed an active balancer.
The issue I am facing is the following.
One of the cells shows an important difference from the other cells when charging and discharging.
Here some examples:
1_LI.jpg
In a moment the BMS triggered the cell protection set at 2.50 volt.

The strange thing is that few moments before, the cell was around the same voltage of the other 3, and so it did after sometime, going back to a similar voltage:
2.jpg
Finally, when I enabled the charger:

3_LI.jpg

The BMS was triggering the overcurrent protection, so I set the voltage to 4.00 temporary.

I suspect my cell is broken, but i would like to know if anyone has some better idea.
Thanks!
 
Hello everyone.
I searched a lot in various forums, but it seems I wasn't able to find a situation similar to mine.
I have a system with 4 lifepo4 cells, 280 AH, bought on AliExpress 1 year ago. I have a PEYO BMS which is a DALY clone.
I updated the firmware to version 210629.
Initially to balance the cells, as I don't have a suitable home charger, I left them connected in parallel for various weeks.

For future reference, this does absolutely nothing to "balance" cells. It merely equalizes their voltage.

I also installed an active balancer.

This should help, but they can be overwhelmed by the charge/discharge current.

The BMS was triggering the overcurrent protection,

Over current or over-voltage?

so I set the voltage to 4.00 temporary.

This is an extraordinarily bad idea.

I suspect my cell is broken, but i would like to know if anyone has some better idea.
Thanks!

The behavior you have observed is indicative of:
1) loose connection or poor ring terminal crimp.
2) increase in cell internal resistance (cell failure)
3) loss of capacity (cell failure)

EDIT: Also good to confirm that the BMS voltage readings are accurate with a separate meter.
 
Last edited:
@colevr , half the voltage issues seen on the forum are due to bad connections. Verify that the bus bars on that cell are making good contact with the cell terminal, all surfaces are clean and that all fasteners are tight.

It could be a balance wire issue or a cell issue also. Move the problem cell to a different position. If the voltage issue follows the cell then you likely have a bad cell. If the problem stays at that position then it's a balance wire or BMS issue. Check the crimp on the ring terminal.

Top balancing before putting the cells into use could have alleviated this issue. It's something that you should have done and still should do.
 
I agree with the likelyhood of there being a bad connection.

I would clean / burnish all connections thoroughly and re-assemble with the bus bars in the same locations but that one cell in a different location .... then retest.
 
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