Cell failure or what? What do you think?

[ale.agos]

Hello everybody and thanks for your attentions and/or suggestions.
since last month of November, due to the fact that hours of sun are less and weather has been cloudy for more days than usual, I started to notice a problem with one of cell of mine 12s - 36V Lifepo4 battery bank.
That cell's voltage (cell nr. 3) is lower than others and sometimes, according to BMS reading, is totally wrong (very low) so seem that the cell is KO.
This happen in the late evening when SOC goes at the lowest level and BMS stop the battery's discharging.
Opposite, when SOC is high cell's nr. 3 voltage became similar or equal to the others...
I attach some files to show the situation.
So do you think that cell nr. 3 is totally KO or maybe I can resume it? Maybe disassembling battery bank and doing again top balance of all cell?
Or maybe is the BMS who is not working properly? I said that because, according to set-up, when cell nr. 3 voltage goes low it should stop discharge of battery but that doesn't happen... It stop discharge but when cell nr. 3 il lowest than value that I put in "cell volt low protect" and also "diff volt protect2" seem is ignored in that case.
Hope to be clear and sorry for my English!

 

[RSahan]

Check the cell busbar connections to see if its loose

 

[sparrowhawk]

Sounds like maybe it's gotten out of balance. Did you do a top balance before you started using the batteries? If so then do another top balance on them.

Sorry I missed the part where you did a top balance, so yes do another. Or you could set your power supply to between 3.4 & 3.65 volts and charge just that one cell and see if that works.

 

[Pim57]

You can swap cell 3 with another cell and see if the problem moves with the bad cell

 

[ale.agos]

Check the cell busbar connections to see if its loose

I don't think that could be the solution because I tested the voltage of cell 3 with a multimeter and voltage value was the same of that one prompt by BMS app. So I think the connection is ok.

 

[ale.agos]

Thanks for suggestions. I think that I will do a new one top balance and I will put together the battery moving cell 3 to another position... I'm also thinking to use an active balancer with the goal to keep cells better balanced. First at all I will try with "top balance", if I will not work and I will need to replace the cell, I will add the active balancer also...

 

[sparrowhawk]

Thanks for suggestions. I think that I will do a new one top balance and I will put together the battery moving cell 3 to another position... I'm also thinking to use an active balancer with the goal to keep cells better balanced. First at all I will try with "top balance", if I will not work and I will need to replace the cell, I will add the active balancer also...

Keep in mind that most active balancers try to balance all the time which has been shown to cause the batteries to become even more out of balance. There are some that can be programed to turn on and off when desired and appear to work great. Otherwise it will be up to you to turn it on and off at the appropriate times to keep your batteries balanced.

 

[400bird]

I don't think that could be the solution because I tested the voltage of cell 3 with a multimeter and voltage value was the same of that one prompt by BMS app. So I think the connection is ok.

So, you measured and verified the 0.2 volts on cell 3?

I think you also said that cell 3 showed the highest voltage at higher SOC?
If so, I doubt a top balance will help. That cell likely has lower capacity than the others.

 

[RSahan]

I don't think that could be the solution because I tested the voltage of cell 3 with a multimeter and voltage value was the same of that one prompt by BMS app. So I think the connection is ok.

Well, that doesn't mean that the connections are good.

 

[ale.agos]

Well, that doesn't mean that the connections are good.

If BMS measure the same voltage of that one I'd measured with multimeter from cell + and - poles force me to think that connections between BMS and that one cell's poles are OK. I'm wrong in what thinking that?

 

[RSahan]

If BMS measure the same voltage of that one I'd measured with multimeter from cell + and - poles force me to think that connections between BMS and that one cell's poles are OK. I'm wrong in what thinking that?

Check the bus bar connections. Loose bus bar bolt will cause the cell not to charge correctly thus BMS showing lower voltage and same with your multimeter

 

[ale.agos]

hello again and sorry for late reply. I'd checked deeply by disassembling the battery and result has been that cell 3 was KO. As soon as I disassemble the battery I noticed that cell nr. 3 was swollen and its voltage was near to 0... I ordered a new one cell and just right now I'm rebuilding the battery after the top balance of all 12 cells...