Cell health in a 16s pack

[Alexplose]

Hi,

I built my pack using new EVE 280Ah matched cells, didn't top balance at first but used the bulk charging of my mpp-solar inverter, saw that cell #13 was always the lowest, and couldn't charge over 3.4v.
Here are some JK-BMS app screens before top balancing :
A 0% lowest is #13 =============== Next day at 100% ============ During normal use cells seems balanced



So I disassembled the pack and then top balanced them in parallel, rested overnight, and plugged it back in my inverter.



The cell #13 is always the lowest voltage as shown.
Between the 1st build and 2nd build after top balancing, the busbars have been shuffled, and nuts have been tightened again, and still #13 the issue, can anyone tell me the issue ?

Could this be a terminal oxydation on this particular cell ?



Yesterday when charging using solar, the #13 was the highest voltage but still the lowest on load.



If someone has any clue, cells are a month old ?

Regards,

 

[sunshine_eggo]

So I disassembled the pack and then top balanced them in parallel, rested overnight, and plugged it back in my inverter.

Please elaborate.

What exactly did you do?

 

[Alexplose]

I let the mpp-solar charge my pack to floating voltage. Then I re-assembled them in parallel, and then charged them using a server power supply and a converter to adjust to 3.65, charging at 4A.
Checked regularly the voltage, voltage went up to 3.8v because of my bad quality converter, but not for long, when disconnected, the resting voltage next morning was 3.5v.

Assembled back in a 16, and started using it normally again.

 

[Alexplose]

When charging, the #13 cell is the highest voltage

 

[Supervstech]

Simple answer, #13 is always lowest v on discharge and always first to reach cutoff when charging… the cell is lowest capacity.
It is not the same capacity as the rest.
Either it is low, or ALL the rest are high…
What KWh are you getting from the bank?
You can think of it in two ways… either you got screwed on that cell, or you have a built in capacity saver for the rest of the bank… but that cell will die sooner than the rest. I would adjust my charge parameters to keep that cell out of the max discharge parameters and let it go, unless the bank is seriously underperforming because of it.

Unless I could get a replacement cell easily.

 

[Steve_S]

Just a thought for process of elimination, take cell 13 and swap it with say cell 6, charge & monitor and see what happens.

 

[Alexplose]

Thanks for your answers, I'll try the swap solution, and get back with the results

 

[Alexplose]

Here are the results, I swapped bad cell #13 with #09, as you can see cell #09 formerly #13 is behaving the same way.
Also all the cells start dropping at the same time, but this cell drop faster, is this due to internal resistance that already build up ? or really an under capacity cell ?


What should I do ? does this prove anything if I send this to my alibaba supplier ?

 

[sunshine_eggo]

What stands out most significantly to me is that for 3 days, you're operating the battery at a very low state of charge.

#9 is definitely the low outlier, but it's the outlier when all 16 cells diverge. The only thing this says is that #9 (formerly 13) is the lowest capacity cell.

 

[Steve_S]

This simply verified that the Suspect Cell is indeed the weakest. So next is to deal with the vendor to get an exchange / replacement but now you can show that this one cell is misbehaving. You cannot "fix" it, it is what it is and not repairable in any way. The only options are to swap the cell with a good one, or to limit the [pack to what that cell can do. The CATCH, that will be the limiter for the entire Bank, so if you have 5 packs in a bank, none will ever be able to go past the top voltage of the weakest cell in the entire bank..

A single 280AH cell is 5.2kg so even if airfreighted is not "insane" (All S&H is insane right now, so relatively not insane) if worse comes to worse.

 

[sunshine_eggo]

This simply verified that the Suspect Cell is indeed the weakest. So next is to deal with the vendor to get an exchange / replacement but now you can show that this one cell is misbehaving. You cannot "fix" it, it is what it is and not repairable in any way. The only options are to swap the cell with a good one, or to limit the [pack to what that cell can do. The CATCH, that will be the limiter for the entire Bank, so if you have 5 packs in a bank, none will ever be able to go past the top voltage of the weakest cell in the entire bank..

A single 280AH cell is 5.2kg so even if airfreighted is not "insane" (All S&H is insane right now, so relatively not insane) if worse comes to worse.

I respectfully disagree with your conclusion this cell is bad.

Evidence presented thus far:
1) pack imbalanced (see image below)
2) Cell #13/#9 is not fully charged (see image below)

3) Cell #13/#9 drops off first.
4) There is some evidence #13/#9 is also the highest during charge, but that isn't consistent with the voltage snapshot or the BMS IR readings.


Frankly, after re-examining the initial data, I'm not at all confident that #13/#9 is either bad or the lowest capacity.

Pack is notably imbalanced.

Top balance pack and re-evaluate.

 

[Sojourner1]

What are your charging settings? You mentioned solar how much? When does balancing happen?

 

[Alexplose]

I used to balance at 3.3v, but when I saw that the pack was unbalanced I lowered the setting.
I have a 3.3kW solar panel array, what are the settings you need ?

 

[Sojourner1]

Your charge settings?

 

[Alexplose]

Here are my current settings.

 

[Sojourner1]

That's for the bms, correct?

What about your SCC or 120v charger? Bulk, Absorb,Float? Time in absorb if using it?

 

[Alexplose]

Here are my inverter settings, sorry it's in french, but settings are standard so easy to understand I hope.

 

[sunshine_eggo]

I used to balance at 3.3v, but when I saw that the pack was unbalanced I lowered the setting.
I have a 3.3kW solar panel array, what are the settings you need ?

There is no such thing as "balance at 3.3V." "Balance" occurs in the extreme upper or lower leg of the voltage curve where the SoC is the same.

Cells at 3.3V can be at wildly varying SoC.

 

[Alexplose]

Here is the charging curve I get



I'll let it charge today, and see if everyone is going up.

 

[Alexplose]

Should I start a bulk charge ?