How much cell variation is "acceptable?

[harpo]

Ok, after months of accumulating all my components for a remote cabin solar electrical system I finally not only have all the components but I now actually have my cells {280AH variety) from Amy (???). They arrived 3 days ago only 107 days after ordering them. Of course, I knew going into this that it would take months and I allowed for it. I must say they arrived supremely packaged. I was impressed. Initial voltages revealed that all cells were within a couple hundredths of a volt. All around 3.27v.

I proceeded to top balanced them as prescribed by many members on this forum until the charging amps dropped to .15amps. As a side note, it certainly is true that one can not tell the amount of charge on a cell by voltage (with LIfePo4)-----I top balanced at 2.6v per cell and it took hours for the amps to drop.

I then capacity tested them with a draw of 50 amps. I stopped the test when 283AH had been consumed. Voltage measurements 10 minutes after disconnecting the resistive cabling was done. Most were in the 2.72-2.77v range. One cell was at 2.84v and another was 2.66v.

My question is this; what voltage variation is "acceptable" with a high of 2.84v and a low of 2.66v? I know that no one cell will be identical to another and thus I expect some discrepancy. I just don't know what variation is "acceptable" and what is "concerning"?

Overall I have to say I am rather pleased with the cells received from Amy. They certainly were not the least expensive option for 280AH cells but it seemed as though purchasing from her was less of a risk of a bad experience than with some other sellers.

Looking forward to hearing answers from the members here.

 

[MisterSandals]

My question is this; what voltage variation is "acceptable" with a high of 2.84v and a low of 2.66v?

Since you top balanced them, they are not expected to be balanced at the bottom. I would charge them back up and see how they hold their charge. I would redo the top balance on them after the beating you've given them.

 

[harpo]

Thank you for the reply. Was it not the appropriate thing to do to capacity test? I sense you think I was too hard on them in so doing.

 

[John Frum]

Thank you for the reply. Was it not the appropriate thing to do to capacity test? I sense you think I was too hard on them in so doing.

No its fine.
Especially since you discharged them as a pack with bms protection and stopped before the first one hit 2.5 volts, right?
Personally I would have discharged until the weak cell hit 2.5 volts that gives the true pack capacity.
I suggest to charge them up and see how well they reconverge at the top.
If they don't reconverge to your satisfaction then you can top off the cells individually to re top balance them.

 

[John Frum]

During normal operation I like to charge my pack well into the high knee so that the bms gets a chance to tweak the balance.
I typically only discharge to the point where the weak cell drops away from the rest of the pack.
Over many cycles that will cause a little more stress on the weak cell making it even weaker.

 

[MisterSandals]

Was it not the appropriate thing to do to capacity test?

I think it helps people decide if they got what they paid for and helps make a timely claim if available (most are pretty much as-is regardless of the effort).

I sense you think I was too hard on them in so doing.

A lot of folks have ruined their newly acquired cells this way as their first experience with LiFePO4 cells.

Especially since you discharged them as a pack with bms protection and stopped before the first one hit 2.5 volts, right?

I did not consider this and the discharge method is unclear.

I'll go with everything smoothJoey said.

 

[harpo]

Yes, I stopped the test when the AH's exceeded 280 (283 actually) and all cell voltages at that time were above 2.6v as stated above.

I just want to know if I got good cells capable of 280AH. I did so I stopped the test, unhooked the resistive load cables from the battery and commenced charging the pack back up with a Victron I22 30amp charger. For now I just used the pre programmed LifePO4 setting on the charger.

I think once the charge is complete I will let them settle and check each cell for voltage variation. At that point, what variation do think I should do a top balance again before putting them into service?

 

[MisterSandals]

For now I just used the pre programmed LifePO4 setting on the charger.

No BMS? LiFePO4 charger cannot monitor cell voltage unless it has cell level balance leads,

 

[John Frum]

Yes, I stopped the test when the AH's exceeded 280 (283 actually) and all cell voltages at that time were above 2.6v as stated above.

I just want to know if I got good cells capable of 280AH. I did so I stopped the test, unhooked the resistive load cables from the battery and commenced charging the pack back up with a Victron I22 30amp charger. For now I just used the pre programmed LifePO4 setting on the charger.

I think once the charge is complete I will let them settle and check each cell for voltage variation. At that point, what variation do think I should do a top balance again before putting them into service?

You want to see the delta from the time the pack enters the high knee until the charge terminates and after they settle.
I would be satisified if they all stayed above 3.45 volts at 48 hours rest.
But for it to be a fair test you want the pack to be charged to at least the level that you will charge to under normal operating conditions.
If the bms trips on a high cell then the resting observation may be invalid.

The details of the charge profile matter as do the bms settings.
What is the high cell cutoff on the bms and what is the bulk/absorb voltage and what is the configured current?

 

[John Frum]

I like to charge to 3.55 volts per cell.
That leave a 100mv headroom but will still charge the cells very full and give the bms a chance to balance the pack.
If the bms doesn't trip when charged to 3.55 volts per then I would see if the bms can whittle a couple of millivolts off the delta on each cycle.
As long as it gets closer under normal cycling then you are good.
If it gets worse then you need to top off the cells again.

 

[RV10flyer]

283ah is exactly what most cells from SLT that I have tested at.