How low is a safe voltage for a cell?

The saga of Battery 136 continues. In a previous post I recorded cell 16 charging to an excessive voltage and causing erratic behaviour with my hybrid invert/battery system. GE responded by swapping my primary and secondary batteries over, and replacing the BMS in battery 136 and the over-charging stopped.
Now another cell in the same battery pack is playing up... cell 13 drops abnormally low during heavy discharge. This got worse last night when cell 13 voltage crashed after a short charging session (this is new behaviour) and left the battery SOCs out by over 25%. I started a calibration to bring the two batteries into alignment and cell 13 in battery 136 is now down to below 2.3V...

Just how low can a Lithium Iron Phosphate cell go before it's irreversibly damaged?

Ooo that's low.

Does it go high on charging?

I had an issue on cell 13 when the block of 4 interconnect bus bar was loose.
2.5V is the minimum I think

I had a cell (cell 16) in the same cell block that was going over-voltage when charging (it hit 3.7V regularly) and an engineer came out to replace the battery management board and swapped the battery to be the secondary of the pair. That changed things enough that cell 16 only goes over voltage occasionally now.

The cell 13 problem is the same pack of 4 cells, but should not be a loose connection because an engineer has already looked at it.

After the recalibration finished the cell voltages have been only mildly different, so even though I contacted my supplier and we both asked GE to replace the battery pack, they consider the fault fixed. I'm not convinced... my data shows cell 13 as still the bottom of the spread of voltages and now the cell 16 issue is re-surfacing...

I'm not sure what the trigger is for GE to replace the battery pack, but at least I have evidence of having reported the issues right from the start.

Pete.J that looks quite a substantial difference, 200mV?

Do you see the divergence when the battery is full or empty, under load or idle?

Pete.J back in the summer when ge were answering the phone I had a similar issue with cells variation, I was told that they don't consider the variation a problem until it reaches 300 mv or adversely affects the battery performance in terms of capacity.

Cell 16 goes over voltage when charging - especially if charging the battery continues to charge at 100%. Cell 13 goes under voltage during deep discharge. Both usually show more than 200mV difference, and sometimes bigger (see my first post).

The battery pack still works, which is probably why it's not been changed. But, as an analogy, it feels uncomfortably like driving with a grumbling wheel bearing - things work (mostly) at the moment but you're not quite sure when the wheel will seize or fall off. I've always preferred to repair or swap out components that are obviously failing BEFORE they break and do damage.