Help one cell charging discharging to fast

[Solar123456]

Help! I am working on precharging before too balance and one cell keeps triggering high voltage disconnect.
I disassembled battery and discharged that cell a bit to get it closer so I could o recharge them a bit before top balancing in parallel with 10 amp charger but it discharged really quickly. Few minutes with 150 watt heater capacity tester.
The voltage was then matching other so I reassembled with bms and started precharging and it jumped ahead of others again quickly and triggered disconnect.

is this normal? Should I just start top balancing with charger or is something off?

8s 24v 280 ah Eve cell battery with JBD 150 amp bms

 

[time2roll]

While fully assembled I would attach the capacity tester to the cell that has higher voltage. Then start the series charge. Use the tester to reduce the voltage of that high cell as needed to keep in line with the others. Might be easier if you can turn the wattage down a bit.

 

[koobs]

You'll see large voltage fluctuations at that higher voltage range past 3.4 due to the properties of this chemistry. That cell is likely to simply be at a higher SOC than the rest. Nothing to worry about yet. See graph below for evidence.
If you're at the stage where you are bringing all the voltages up in series before top balancing, then unplug that high cell and continue to charge the pack to 24.5v instead of 27.2. (depends on how customizable your BMS is).
Make sure the BMS continues to be connected.
Otherwise if not customisable then just split the pack, parallel it and commence top balance.

LiFePO4 chart taken from the forum post here by jasonhc73

 

[OM617YOTA]

Thank you everyone. I'm at exactly the same point, doing exactly the same thing. Just reconfigured my pack to parallel and am starting the final top balance, and while babysitting the pack I jumped on the forum to make sure I was doing it correctly.

 

[TomC4306]

I am also at this stage. I configured 4 each 12v packs and using a 10 amp supply charged to 3.45. Above about 3.4 each 4 pack had a runner. I didn't let the BMS cut off charging however. I watched carefully and when the runner went above 3.6, i terminated charge and put that 4 pack into parallel. Then went on to the next 4 pack. Once all 4 were paralleled, i charged to 3.5. This process took all week. I just now paralleled all 16 cells and put them back on the charger at 3.6.

 

[Solar123456]

You'll see large voltage fluctuations at that higher voltage range past 3.4 due to the properties of this chemistry. That cell is likely to simply be at a higher SOC than the rest. Nothing to worry about yet. See graph below for evidence.
If you're at the stage where you are bringing all the voltages up in series before top balancing, then unplug that high cell and continue to charge the pack to 24.5v instead of 27.2. (depends on how customizable your BMS is).
Make sure the BMS continues to be connected.
Otherwise if not customisable then just split the pack, parallel it and commence top balance.

LiFePO4 chart taken from the forum post here by jasonhc73
View attachment 61248

If I unplug the high cell do I need to reorder the bms cell order or can I just disconnect that one and adjust the charger / bms # of cells?

using JBD 150 which is similar to overkill bms

 

[MisterSandals]

If I unplug the high cell do I need to reorder the bms cell order or can I just disconnect that one and adjust the charger / bms # of cells?

No, you cannot change the number of cells your BMS supports. It looks like your cell in position 3 is a lower capacity. Make sure you label your cells so you can keep track of them, particularly the one in position 3.
You're probably force to put your cells in parallel at this point and finish your top balance. Hopefully a top balance will bring it in line with the other cells to make it useful.

 

[Solar123456]

No, you cannot change the number of cells your BMS supports. It looks like your cell in position 3 is a lower capacity. Make sure you label your cells so you can keep track of them, particularly the one in position 3.
You're probably force to put your cells in parallel at this point and finish your top balance. Hopefully a top balance will bring it in line with the other cells to make it useful.

I just put the other 7 in parallel to charge them up to the voltage of the questionable cell. I plan on adding that one to parallel once it catches up to others. One thing I have noticed is when I disconnect the questionable cell and test the voltage, it does slightly drop.
It was 3.61 then a few minutes later dropped to 3.60 then a few minutes later 3.59 and now it’s 3.58. Just checked it again and it’s 3.567

is this normal? It’s not connected to anything and I am just using voltmeter to check it periodically. These drops have all been in less than 30 minute and every time I check seems slightly lower

 

[time2roll]

Normal. Same as lead-acid charging to 14.6 will settle to 12.7 volts after resting.

 

[DerpsyDoodler]

I just put the other 7 in parallel to charge them up to the voltage of the questionable cell. I plan on adding that one to parallel once it catches up to others. One thing I have noticed is when I disconnect the questionable cell and test the voltage, it does slightly drop.
It was 3.61 then a few minutes later dropped to 3.60 then a few minutes later 3.59 and now it’s 3.58. Just checked it again and it’s 3.567

is this normal? It’s not connected to anything and I am just using voltmeter to check it periodically. These drops have all been in less than 30 minute and every time I check seems slightly lower

Yes that's normal. The voltage settles. My bets are all you need is a proper top balance and you'll be good to go. unless that cell actually is a different rated capacity.

 

[Solar123456]

I’m wondering if there is a way to speed the process up? I tried discharging the cell with higher voltage but when I hooked it back up it jumped ahead again. My main concern is I only have a few more days to finish testing before I run out of the 15 day window to dispute if the cell ends up being bad. Any ideas?
I only have a 8s 24v bms and 150 watt heater capacity tester. I do have a 12v 1500 watt inverter but would be hesitant to run it without a bms.

 

[TomC4306]

Parallel all of them and connect to your bench top power supply at 3.6v. you're close enough. It won't take long.

 

[koobs]

Agree with above. At this high of a state of charge, it wont take long to complete your top balance with all cells in parallel. Use two power supplies or more if you have them, to get the current higher

 

[Solar123456]

So I have been top balancing the cells for a few days and the power supply doesn't seem to go all the way down to zero but get close.

I have it currently set to 3.63 volts and am maxing out the current and the voltage on the power supply will bounce between 3.61 and closer to zero current for a few seconds but the voltage will jump to 3.62 or 3.63 with about 1 amp of current. Its been bouncing around like this all morning.

I also measured the voltage at the cells and all 8 cells are either at 3.621v or 3.622v.

Should I call this balanced or do I need to wait until the power supply current goes to 0 at 3.63v?

 

[koobs]

Sounds pretty balanced.
Not sure whats going on with the bouncing though. Shouldnt do that. Does it bounce only when you touch the setup or move leads? Perhaps a poor connection

 

[Solar123456]

I thought that was odd. No its been doing it the entire time as its left alone. No movement or messing with anything. I checked all the connections and they are tight. The voltage just keeps changing on the power supply bouncing between 3.61 and 3.63 and the current increases with higher voltage.

 

[MisterSandals]

I thought that was odd. No its been doing it the entire time as its left alone. No movement or messing with anything. I checked all the connections and they are tight. The voltage just keeps changing on the power supply bouncing between 3.61 and 3.63 and the current increases with higher voltage.

2/100ths of a volt is probably less that the sampling error of the components as a whole. I would consider that just noise and nothing meaningful. Its annoying for a digital display to bounce around though.

 

[Solar123456]

So I dissembled them as I am going to put them in series with bms for capacity test. Disassembled I noticed the voltage of each cell dropping which I understand is normal as it approaches a resting voltage but I am noticing a slight difference in cell voltage.


Cell Voltages:
1) 3.608
2) 3.612
3) 3.610
4) 3.610
5) 3.610
6) 3.610
7) 3.610
8) 3.615

Is this normal?

 

[Brett V]

I just topped mine out last night. Even at 3.65, there's a little current and the voltage was dancing around a tiny bit.

 

[Brett V]

So I dissembled them as I am going to put them in series with bms for capacity test. Disassembled I noticed the voltage of each cell dropping which I understand is normal as it approaches a resting voltage but I am noticing a slight difference in cell voltage.


Cell Voltages:
1) 3.608
2) 3.612
3) 3.610
4) 3.610
5) 3.610
6) 3.610
7) 3.610
8) 3.615

Is this normal?

I left them in parallel and disconnected the power supply overnight to allow them to further equalize.