How to balance LFP in series?

[Michael9dk]

Background:
20 x EVE LF105 in serie.
They have never been top-balanced.
Discharged op to 60A between charges, for 6 months without issues.

Then one time, it was discharged 74A (3.199V), and I drowe until the BMS shutdown at 83 Amps. I had no other option to get home, and engaged Emergency-mode (well aware that I would fry a cell or more).
One cell was down to 0.7V at light load (and was replaced when I got home).

But what I really noticed, is that about 50% of the cells, always have been out of balance (150-200mV and more), during charge.

My JK-BMS has been balancing with 0.6A for about 48 hours with no significant difference...

I'm waiting for a 2x60V/10A power supply to arrive, and intend to charge the battery with a lower voltage and a very low current.
Would that work if you have the time?

How would you balance a 20-cell series battery WITHOUT disassembling it?

 

[dominik h]

Hook up a charger and let the BMS do its job.
In 48h the BMS can only transfer about 24Ah.
If your battery is completly debalance it will probably take much longer.
But it is also possible that same cells are damaged and have a higher self discharge.

 

[Michael9dk]

My charger turns off when the BMS hit OVP, and need to be unplugged to start again.

Once half the cells reaches 3.65V, the rest is about 3.40V.
After 15 hours rest with balance (>5mV diff. @3.34V) they all settle at 3.333 to 3.337V.

Turning the 18A charger on again, it takes like 10 minutes to hit OVP again with a cell difference of 270mV.

So I'm thinking that floating at 1A/69V (3.45 cell) would let the balancer do its work without hitting OVP instantly?

 

[Zambam]

Sounds like you have more bad cells? To find the bad ones, check each cell's capacity and internal resistance. You can do that w/o disconnecting the series connections. You will need a single cell charger and a battery capacity tester.

edit: Does your EVE LF105 cells have screw terminals. If so, check all connections for corrosion and tightness. That's a big battery! 6.7 KWh! What's it used in? Any photos?

 

[Michael9dk]

I really hope not. They're only 6 months old with ~15 charges. Original EVE from Nkon.

Checked my screenshots of the first charge, and they were off by 220mV, when the charging finished.

My guess is the cells are from 2 different batches. Looking at the deep discharge, SoC were about 20Ah apart (2.66 vs 3.18V at 82Ah discharge).

I'll do the slow charge and see if they will even out, first.