Overcharged cells...

[FLBikeDude]

So I have this battery made with Headways...
16s,2p - 38120L cells

The pack has been "fine" for over 3 months, when all of a sudden a group of parallel cells just crapped out.
The cells have been previously charged about 50 times with no issues whatsoever.. i.e. Perfectly balanced to 3.60-3.68v each..
I've hit LVC on the BMS only twice and the cells were still very well balanced (within .2v of each other, e.g. The last time I hit LVC, ALL the cells were in the 2.7 - 2.9 range) 20.3 AH used at 1C or less 15-18a

One group of cells died (2p) so I removed them...
I ordered 2 new cells.. Should be here in about a week..

In the mean time I am charging with a 12v Charger... 2a

Well last night I wasn't paying attention and one 4s group of cells were all charged to 3.9v (charger makes a small noise when it shuts off (normal operation) at 15.6v and I immediately checked the voltage on the cells and they were all at 3.85 - 3.90v and all dropped to 3.60-3.70 within a min or so...

The only question I have is..

How badly did I damage the cells..

I know the charging drops from 2a to about 650ma when it hits 14.8v

[liveforphysics]

It would just accelerate decomposition of the electrolyte for a short period while at higher than normal voltage. LiFePO4 is pretty durable to the event you described, my guess would be no observable damage caused at all.

[Trackman417]

It would just accelerate decomposition of the electrolyte for a short period while at higher than normal voltage. LiFePO4 is pretty durable to the event you described, my guess would be no observable damage caused at all.

So, the maximum voltage of a cell is determined by the electrolyte then?

[liveforphysics]

It would just accelerate decomposition of the electrolyte for a short period while at higher than normal voltage. LiFePO4 is pretty durable to the event you described, my guess would be no observable damage caused at all.

So, the maximum voltage of a cell is determined by the electrolyte then?

For LiFePO4, it's not really storing more energy as as you continue to increase cell voltage (unlike many other chemistries), it's largely just electrolyte stress in this situation, and fortunatly, even very poor quality electrolytes handle 4v quite well. It's rapid geometric increases in electrolyte stress as you go up from there. Hence the large difference in a cell puffing sitting at 4.2v that doesn't puff at all sitting at 4.125-4.15v.

[Hillhater]

Perfectly balanced to 3.60-3.68v each..
.....I've hit LVC on the BMS only twice and the cells were still very well balanced (within .2v of each other, e.g. The last time I hit LVC, ALL the cells were in the 2.7 - 2.9 range)

You know 3.60-3.68v is not really "perfectly balanced" ..
.. Even just a good balance should be within 20mV.
Sure, you can tolerate 80mV, but if you are looking for a cause of your problems, it would be wise to start with a better balance situation.
... And that 0.2v is not too flash either !