One cell level too high - LiFePO4

[Awsmits]

According to my BMS, I have one cell of of an 8S LiFePO4 battery that is significantly higher then the other seven (3.6 vs 3.35V at full charge). Other than doing a discharge then top balance all 8 cells, is there another method that would get this cell back in line? Would a discharge of just that one cell work?

 

[Jason.Blier]

You could discharge the one cell with a power resistor or automotive headlight bulb. That works quite well to bleed power from a high cell.

 

[MisterSandals]

is there another method that would get this cell back in line?

How about looking at figuring out how/why it got out of line?

Any history of the cells? All new? All started at same voltage?

Happen to have a pic of how its wired? Its surprising how many issues are found/resolved with a pic...

 

[Awsmits]

All new in October/22. I wired the 8 cells in series, added a JBD 100A BMS, pretty sure I got the balance leads correct. All cells started at 3.29 +/- 0.02V. I did not top balance, since the voltages and resistances were so similar.

 

[HermanD]

According to my BMS, I have one cell of of an 8S LiFePO4 battery that is significantly higher then the other seven (3.6 vs 3.35V at full charge). Other than doing a discharge then top balance all 8 cells, is there another method that would get this cell back in line? Would a discharge of just that one cell w

I have the same problem. in 16 S setup. the 15-16 always go red 3.65. not sure why. even I add extra lenng of cable wire to the Red it help a little.

3 days ago I moved into the garage and separate them out the EG4 48v battery rack x3 and the 16s 302 amp pack both connect to the Busbar. I used extra connection Bar between the batteries that Amy Zheng gave me which mean double it. this help better. in balance.
Other way you can do is buy the active balancer on Aliexpress here: https://off-grid-garage.com/battery-management-systems-bms/ check out his link below with the Active balance Neey or the one below that cheaper. That help drain the battery high voltage bring to the lower voltage. Another problem I think is that the solar too big or high charge and the batteries does not have well connection cause it.

https://www.youtube.com/@evempsolar

Here is my problems ... maybe it's the same as your. I am new DIY... any idea is really appreciated.

 

[sunshine_eggo]

All new in October/22. I wired the 8 cells in series, added a JBD 100A BMS, pretty sure I got the balance leads correct. All cells started at 3.29 +/- 0.02V. I did not top balance, since the voltages and resistances were so similar.

This is why. Identical voltages at reduced SoC are meaningless. Voltages only hint at "balance" when they are all north of 3.45V and even then, you can have a runner.

My 9 cells were all within 0.001V of each other with identical IR, but they ranged from 28-41% SoC as received. That's 36Ah difference on 280Ah cells. That's about 1-2 months of passive balancing 24/7 before that BMS can bleed off 36Ah.

I've seen as little as a 0.1% difference in SoC cause a runner

 

[HermanD]

Yeah, you needed Active balance... I remember now. The active balance help at top and low stage of charge too... my battery is really in different voltage and SOC when it low. one cell is down to 2.6v and other still 75% 3v or 3.1v, even though it show full at the same volt or in different less than 0.008. But when discharge it show as above. The bank is 48v 16s 302 amp CATL max is 315 I can set it too but when it drain empty which mean BMS shut off. it show only 250 amp. So this mean the cells not fully balance capacity in amp. The Active balance that would bring the high volt or SOC over to the lower battery would be great to buy and have.

 

[Bleedingblue]

You could discharge the one cell with a power resistor or automotive headlight bulb. That works quite well to bleed power from a high cell.

Any threads on how to build one? You would think it needs 12 volts to work properly

 

[mikefitz]

Obtain a vehicle headlamp filament bulb, rating about 50 watts . Connect two cables to the bulb . Strip the insulation from the other ends so you have two 'probes'. Whilst the battery is under charge and monitoring the cell voltage, idealy all cell volts being displayed apply the probes across the high cell for several seconds. After a little practice and adjusting the time the high cell is loaded, its possible to bring cells in line.
If things are changing too fast lower the charger voltage. Other cells will become the 'high cell' thus you will need to play 'wack the mole' with the discharge bulb.

Mike

 

[Bleedingblue]

Obtain a vehicle headlamp filament bulb, rating about 50 watts . Connect two cables to the bulb . Strip the insulation from the other ends so you have two 'probes'. Whilst the battery is under charge and monitoring the cell voltage, idealy all cell volts being displayed apply the probes across the high cell for several seconds. After a little practice and adjusting the time the high cell is loaded, its possible to bring cells in line.
If things are changing too fast lower the charger voltage. Other cells will become the 'high cell' thus you will need to play 'wack the mole' with the discharge bulb.

Mike

I got a spare cell ill experiment on lol

 

[MisterSandals]

I got a spare cell ill experiment on lol

You can do this with a dumb soldering iron too. Should be around 30W. It does not care if its AC or DC to get hot and bleed off power. Incandescent light bulb works too, may not get bright but it will drain power.