Have I stuffed up this LiFePO4 cell?

[RLP]

Greetings all,

Today one of my prismatic LiFePO4 cells (EVE LF105) was knocked over and landed on a metal object, which left a couple of notable little dents in the large side of the cell, as pictured. I measured that the larger of the two dents is 0.7mm deep, and around 2mm in diameter.

The dented area of the cell didn't get hot as far as I could tell. The cell voltage also hasn't changed - it still matches my other cells to within 0.001V.

I know there is probably no definitive answer to my questions, so I'm just looking for opinions from those more experienced with the construction of these cells, and anybody who has seen similar things before: what do you think? Is it probably safe to use? Is it likely that I'll see a performance degradation later on in the future?

The cell will be voltage and temperature monitored and balanced by a BMS, so I will be able to pick up performance degradation if it occurs.

If the verdict is that I should start planning to replace the cell... what I'm wondering is, is it feasible to drop in just one new cell, into an existing ~1 year old series string of hardly used cells, or is that asking for trouble (i.e. imbalance etc.)? The string is 72V, so I'd definitely be looking for alternatives to replacing/repurposing the other 23 cells!

Many thanks,
RLP

 

[obd.tech]

I've seen other users that have shown bigger dents than that and they have continued to use the cell with no detrimental effects.
Caution is good, but I think these LFP cells are pretty forgiving?

 

[Steve_S]

That IS a problem, those dented places WILL cause issues over time, that will introduce pressures on the super thin foils inside the cell and likely affected the paste as well. Consider that cell DEAD ! Any dent besides a surface blemish can be problematic, a deep dent like that did squish the jelly roll.

Yes you CAN replace just the one cell. Match the cell so if it is an EVE-105AH replace with an identical one.. BUT ensure you get a proper Grade-A cell so that it is in top condition, that way it will not introduce a "dawg" into the mix, If it is better than the existing cells, it will not have a bad effect, but a B or Bulk cell may be more problematic if it drags down the pack.

Prior to ordering a replacement, it would be good to cross check all the cells with a device like a YR1035+ Cell Tester. Get all the cells to identical voltage, then test each cell for their IR (Internal Resistance) so you know what each cell's IR is, some Quality Goo Vendors could then try to match up a replacement cell with a similar IR.

NOTE: Many reseller/vendors ONLY test Voltage & IR at the storage voltage they have received the cells at.. The DO NOT Properly Match & Batch cells. Only a few offer proper A Grade cells with a factory report on the cell showing the full test values.

 

[RLP]

Thanks for the replies.

I too have seen some reports of people using damaged cells without consequence, so it's tempting to try - but what Steve S says does make sense, the safe bet is to assume the cell is compromised.

Since I'm curious and this is more of a R&D setup at present, I will probably give it some cycles in a controlled environment (and report back eventually). I'm guessing the most likely consequence would be elevated self discharge and/or the possibility of an internal short...? I can't imagine much of the total active area of the roll is damaged, so the capacity loss shouldn't be huge?

I will of course make some sort of plan to replace the cell, and it's good to know that that's an accepted practice if done right. My options for acquiring cells are not so great here in NZ so I prefer to import in quantity directly from China, but I may be able to get hold of a single cell from one of the local vendors (for a higher price!). I will ask them what options I have for matching it with the existing cells, but most likely I'll have to settle for any new grade A cell, which would hopefully at least have a lower IR and the same or better capacity than the other cells, so as to not drag them down.

 

[Whats_per_question]

@RLP : Not sure where you are in NZ, but give the guy at roar power a call - he has lent me time on his cell testing machine to check 8 cells on a 24v battery for me, and found a bad cell. He might also have spare cells that might be a match for your cells, as i know they have EVE cells in some of their 105Ah batteries.