large delta on one cell after capacity test

abwillingham

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Doing my first capacity test in my first top balanced Lifepo4 build. As the battery gets closer to the end of the test, one cell has started to drop much more rapidly in voltage than the others. You can see the delta on cell 4 is large. The delta was very small for most of the test. Is this normal?

Edit:
Cell 4 caused the test to end after it dropped below 2.5v, which put the battery capacity at 196AH instead of the expected 202AH.
Is that one cell bad?
 

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labeeman

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Doing my first capacity test in my first top balanced Lifepo4 build. As the battery gets closer to the end of the test, one cell has started to drop much more rapidly in voltage than the others. You can see the delta on cell 4 is large. The delta was very small for most of the test. Is this normal?

Edit:
Cell 4 caused the test to end after it dropped below 2.5v, which put the battery capacity at 196AH instead of the expected 202AH.
Is that one cell bad?
Only if you consider 97% of capacity BAD.
 

abwillingham

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I do consider it bad, since I paid for 100%. Are you implying that 97% of new grade A cell capacity is common and normal?
 

smoothJoey

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@snoobler has said to plan for 95% of advertised capacity.

Is normal for the weak cell to nosedive first.
The next thing to do is charge at a representative rate and see if the cells converge again as they approach full.
Also I hope you made a note of the pack voltage at which the weak cell started to diverge.
That will determine where you should set inverter low voltage disconnect.
 

labeeman

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I do consider it bad, since I paid for 100%. Are you implying that 97% of new grade A cell capacity is common and acceptable?
If from a Chinese supplier yes you are lucky it is 97% and not 75%. On another note have you verified your discharger accuracy?
 
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abwillingham

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Ok thanks. I recall Will P. making a big deal that he was “ripped off“ because he only got 116AH instead of 120AH. I took that to mean anything less than 100% was unacceptable. I thought all LifePo4 cells came from China.
I did not note where they diverged. Can I charge them and run another capacity test to find that?
And what amount of delta is the cut off point?

My next question. I was planning on getting 4 more and making a 2P4S battery but using one OverKill BMS.
Do I need to try to pair this one “bad” cell with a similar “bad” cell?

I could probably get the seller (out of CA) to replace the “bad” cell if I asked.

Thanks
 

labeeman

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Ok thanks. I recall Will P. making a big deal that he was “ripped off“ because he only got 116AH instead of 120AH. I took that to mean anything less than 100% was unacceptable. I thought all LifePo4 cells came from China.
I did not note where they diverged. Can I charge them and run another capacity test to find that?
And what amount of delta is the cut off point?

My next question. I was planning on getting 4 more and making a 2P4S battery but using one OverKill BMS.
Do I need to try to pair this one “bad” cell with a similar “bad” cell?

I could probably get the seller (out of CA) to replace the “bad” cell if I asked.

Thanks
That is a definite NO you would pair them with a better cell to even them out if you pair them with another bad cell you bring the pack down to the bad cell cap.
 

smoothJoey

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Ok thanks. I recall Will P. making a big deal that he was “ripped off“ because he only got 116AH instead of 120AH. I took that to mean anything less than 100% was unacceptable. I thought all LifePo4 cells came from China.
I did not note where they diverged. Can I charge them and run another capacity test to find that?
Yes.
Lets us know how well they re-converge when you charge first.
And what amount of delta is the cut off point?
I like ~50mv.
My next question. I was planning on getting 4 more and making a 2P4S battery but using one OverKill BMS.
Do I need to try to pair this one “bad” cell with a similar “bad” cell?
I concur with @labeeman
I could probably get the seller (out of CA) to replace the “bad” cell if I asked.
I don't think it will hurt to try.
Let us know how you make out.
 

Whinny

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It may not be a bad cell, I suggest checking the busbars for burrs and cleanliness. Polish the posts and bar contact area and reassemble carefully. I would suggest using an anti oxidant on the connections. That being said having a runner in a grade "A" advertised product is certainly not unheard of.....
 

Bob B

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It may not be a bad cell, I suggest checking the busbars for burrs and cleanliness. Polish the posts and bar contact area and reassemble carefully. I would suggest using an anti oxidant on the connections. That being said having a runner in a grade "A" advertised product is certainly not unheard of.....
+1 It is also possible you had a bad connection and didn't get a good top balance.

I would mark that cell so you don't loose track of it ....clean all cell terminals and bus bars thoroughly and re-do the top balance ... and test again with the cell in a different position in the pack.
 

abwillingham

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Recharging the battery and all cells look good. The “bad” cell #4 has been a little ahead the entire recharging time.

I will top balance again and check all the connections.
 

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abwillingham

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I charged up my batteries most of the way and then put them back into parallel and top balanced them again.
When I was done I left them sit all day, still in P, and tonight I measured each cell (3.640v) after I removed them from parallel but before I put them back in series.

I hooked everything back up, Victron smartshunt and the overkill BMS. I sanded my bus bars and swapped the #4 battery to the #2 position. This is what it looks like after 5 min with a 30A load. The #2 cell (which is the same “bad cell”) is immediately out of whack.

If I turn the battery off vis the BMS software, all cells go back to the same charge, Pic #2.
 

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ereams65

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Glad you figured it out because I was just about to say “recheck the posts and connections on that cell”. I had something similar occur and it ended up being a burr on the positive post. Took me a bit to find it.
 

minitt

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just noticed this thread. So I am wondering how big of a Cell delta voltage is actually a concern ? Mine (4 Eve 280AH Cells in 12V config) initially showed a delta of 200milli volt then I top balanced like Will showed in his youtube video. Now it came down to 13mv( at full charge). Is that 13mv voltage falls within acceptable range ? Thanks in advance.
 
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abwillingham

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just noticed this thread. So I am wondering how big of a Cell delta voltage is actually a concern ? Mine (4 Eve 280AH Cells in 12V config) initially showed a delta of 200milli volt then I top balanced like Will showed in his youtube video. Now it came down to 13mv( at full charge). Is that 13mv voltage falls within acceptable range ? Thanks in advance.
I think that is probably fine.
 
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