One cell below the others

[Flod]

I charged my battery after winter today. One cell is lower in voltage and temp than the others. Is this dangerous in any way? How do I fix this? The first pic is from like mid in the charge cycle and the second in the end

[AGM]

Originally Posted by Flod

I charged my battery after winter today. One cell is lower in voltage and temp than the others. Is this dangerous in any way? How do I fix this? The first pic is from like mid in the charge cycle and the second in the end

Temperature differences most likely due to BMS bleeding off some charge in cells 1 to 3 in order to bring their voltages down to cell 4's. Temperature sensors are often mounted on the same PCB as the bleeding resistor and therefore measure PCB temperature rather than cell temperature.

If the 4 cells were in balance at the start of winter, this could point to cell 4 having higher self discharge than the others.

You may want to charge the weak cell manually to see by how much it is trailing the others.
Or, just leave it alone and let the balancing circuit do its job.
After a few weeks of regular use the imbalance should at least partially clear, unless the self discharge rate exceeds the balancing circuit's capability which would point to a more serious problem with cell 4.
In case you have multiple cells in parallel, probably only one of them is responsible for the higher self discharge.
But I've seen a whole parallel group of cells go bad after being over-discharged.
What cell voltages did you see before charging?

[Flod]

Hi the voltages was 3.28 andv3.29 after winter storage. I have 8 ea 100ah winston cells.

[AGM]

Originally Posted by Flod

Hi the voltages was 3.28 andv3.29 after winter storage. I have 8 ea 100ah winston cells.

These voltages seem ok.
Did you do an initial top balance before assembling your 12V bank of 200Ah or was this 800Ah?
If not, I guess you probably would have noticed this voltage imbalance during charging before winter?
If you did do the initial top balancing, and there was no voltage imbalance during charging before winter then you most likely have a dud cell in group 4.

You can choose to ignore this and let the BMS take care of it.
But if there's no improvement after a few weeks you probably want to go start looking for the culprit.

In this case, one solution involves fully charging them to about 3.70V (till tail current is less than 1 percent of capacity), then isolate and let them rest for a few hours. The voltage shouldn't drop below about 3.45V on any of them.
Or better yet, charge cell group 3 (known good) as well, in the same way and then compare the open circuit voltage droop with individual cells of group 4.
If there is a dud, it'll show a faster voltage decay for some time after coming off the charger.
Anyone else having experience with weeding out a dud cell, may have come up with their own ingenious way?

[Flod]

It’s 12v 200ah. I did initial top balancing but I’m not 100% sure I did it correctly. I noticed nothing last season and the voltages seemed to stay in line. I’ll try to let the bms handle this first and if that doesn’t work I’ll try one of the other suggestions. Thanks!

[hzcruiser]

Originally Posted by Flod

It’s 12v 200ah. I did initial top balancing but I’m not 100% sure I did it correctly. I noticed nothing last season and the voltages seemed to stay in line. I’ll try to let the bms handle this first and if that doesn’t work I’ll try one of the other suggestions. Thanks!

And please keep us posted.


The temp reading of cell 4 was unusually low, at 8C. Or was it really that cold and the other readings were high due to the balancing power dissipation?

Also, 8C is not far away from 0C which is minimum temp for some LFPs to get charged. Not sure what happens if one tries to charge an LFP that sits at say -2C. Anyone?

[Flod]

So I splashed the boat last week and have been cycling the battery a few times. It seems that the balancing works and the low cell have become more level with the others. Great functionality in the 123bms! Here is a picture from today when charging with 25A

And here is one a little bit later

[CaptainRivet]

Looks great.

You can also just disconnect and put all 4 cells in parallel and let them rest for 24-48h. Like this they balance themself.
Then put them back in series and cycle them a few times.

[CaptainRivet]

Originally Posted by hzcruiser

Not sure what happens if one tries to charge an LFP that sits at say -2C. Anyone?

You damage it permanently and can throw it into trash...

If you are in conditions like that take LTO cells, they are the only one that can handle even -30 degrees Celsius.

[Flod]

I am aware of that the lifepo4 don’t like charging for in freezing temps. When I did this test the outside temp was around 10 C and inside the boat I guess 7-8C. This should be ok as far as I know. I was out sailing this weekend and cell 4 is still
a little below the others but I can notice the bms trying to even it out.

[CaptainRivet]

Put the cells all in parallel and let them rest for 48hours.
That levels the cells.
After that they will need couple of cycles.
If nothing chances, its not far off so you can operate them without any problems. One will be always the weak one.
If the difference gets bigger to 0.2V get yourself 1 new cell and replace as this will take the other 3 down soon.