Unriddle me this. Cells balance shenanigans

[oscar lopez]

Alright. Just spent a week trying to second guess my cells and my last braincell is asking for help.

It's a simple set up of 4s grade A eve cells and a JK BMS with the 2A active balancing.

Don't have a dc power supply to do individual cells so I rely in the BMS for the balancing, and it worked well for a few months as far as i could tell.

The story starts with a new phone, the BMS app on the new phone giving me the classic 'Failure to pair' issue, and genius me figuring i can just keep an eye on the pack voltage via the Victron app and everything will be fine. So basically not on the loop of what's been happening the last month or so.

So i got home one day to find the lifepo4 pack was off. I scramble to find the good ol' phone with the smashed screen, and after some ADB magic eventually get to display the phone's screen in the computer to find this




I started the process of changing and balancing and that's when things get weird.

- Cell 1 always lags behind while charging, by up to 100mV. Even if i stop charging and let it catch up using the active balancing, it drops back when charging again. Once balanced, and if not charging, it remains balanced.
- Cell 2 shoots ahead the other cells when charging by upwards of 50mV higher than the other, drops down back in line when stopping charging.
- Cells 3 and 4 largely stay together voltage wise most of the way through

When i get to the upper end

- Cell 1 voltage stops increasing once it hits around 3.42v, and even starts dropping after hitting that, while charging... Can't seem to be able to get it to go any higher
- Cell 2 voltage stops being ahead while charging
- Cells 3 and 4 take the lead and hit the voltage cut off pretty damn quick

These screenshots are within a few minutes of each other



After stopping charging, cells 1 and 2 settle around the 3.34v mark. Cells 3 and 4 around 3.44v, so out of balance by 100mV

Does any of this, confusing as it probably is, offer any clues of hints as to what could be going on?

Checked all the connections several times and seem good.
BMS leads also seem ok if the resistance values are anything to go by.
Cell voltages measured with a multimeter are more or less as per the BMS/app.
Have not moved cells around yet as it won't be an easy feat.

I'll keep discharging, balancing and charging see if i can get them all to top balance, but anything else i could attempt besides that?

Thanks

 

[sunshine_eggo]

Checked all the connections several times and seem good.
BMS leads also seem ok if the resistance values are anything to go by.
Cell voltages measured with a multimeter are more or less as per the BMS/app.
Have not moved cells around yet as it won't be an easy feat.

I'll keep discharging, balancing and charging see if i can get them all to top balance, but anything else i could attempt besides that?

Thanks

Set balancing to 3.40V, 20mV and disable balance only when charging.

Hold at 13.8V

Stop when all cells are at 3.45V.

The fact that it's changed suggests either a balancing malfunction or a cell failing with excessive self-discharge.

 

[oscar lopez]

Set balancing to 3.40V, 20mV and disable balance only when charging.

Issue i'm having now is that cells don;t remain above 3.4v long enough to balance. Once charging is on cell 3 soon hits OVP with cell 4 not far behind

Hold at 13.8V

Stop when all cells are at 3.45V.

Ok, i'll see if i can force the charger into float at 13.8v.

The fact that it's changed suggests either a balancing malfunction or a cell failing with excessive self-discharge.

Yeah, i thought cell 1 was dead as it was self discharging when i found it low, even with both charge and discharge turned off. But what i thought would be an obvious diagnosis turned out more confusing than i was expecting.

Thanks

 

[sunshine_eggo]

Issue i'm having now is that cells don;t remain above 3.4v long enough to balance. Once charging is on cell 3 soon hits OVP with cell 4 not far behind

All cells don't have to be above 3.40V. ANY cell has to be above 3.40V before it will start transferring charge from it.

Ok, i'll see if i can force the charger into float at 13.8v.

Lower is fine as long as balance is happening.

Yeah, i thought cell 1 was dead as it was self discharging when i found it low, even with both charge and discharge turned off. But what i thought would be an obvious diagnosis turned out more confusing than i was expecting.

Still might be bad, but you won't know until it re-balances and then you observe it to go out of balance again.

Might also be worth moving cells around to confirm the problem moves with the cell.