how high is to high regarding IR for a lifepo4 cell

[1000kw-hunter]

Just as the title is saying:
how high is to high regarding IR for a lifepo4 cell
I found a cell with just two working years with me in my ess with a IR of 147!!!!!Mohm
Not even 100 cycles...
Bought at around 10-20mohm?

Is this cell still safe to use?

With best reards Igor

 

[Menno]

It will still work but the voltage drop will be very high. First question I should ask when given some response is: what is the cell and how is it run? How many S how many P? Max output / cell.

 

[OffGridInTheCity]

In the 18650 case, I had a cells with 4x to 5x the IR of other cells. I built packs with these and put them in series with other (normal IR) packs. The key issue was I couldn't keep them in balance with other packs in series.

I pulled these and made an entire battery of 14s12p cells and the battery worked in a low stress application in one of my APC 3000s. These were Sony G5/G7 with OPUS reporting 200mOhm IR. Typically OPUS reports 40-80mOhm IR on 'good' cells. I understand OPUS is not a reliable IR measurement tool but I'm trying to say that I believe these Sony G5/G7(s) to be ~4x higher than normal IR.

However, the battery above 'failed' (balance went wild all of a sudden) in only 4 years as compared to normal cells that seem OK in year 8 and counting.

My G5/G7 experience does not make a rule but I carry a working opinion (for lack of any better info) that...
1) that hi IR and low IR in the same series should absolutely be avoided for balance issues at least.
2) Making a battery of ALL hi IR is OK for limited stress (low C) situations but not so good for higher C (per specs) situations.
3) Perhaps there's a shorter life span?

 

[Menno]

Hi IR for me for a 18650 cell is above 30 mOhm. For cell going in my 400P packs even above 20 mOhm is high. But i use high discharge cells just because I have them. The intention of that pack is to run a 5 kW inverter. now I have 40*400 packs to build 3 13S 400P pack that run peak 200A. normal load <100. each 400P pack is 800 Ah and cells are rated each for 25A output. I think that even a bad high IR cell will have some issues.

I was thinking when using an e.g. 320Ah LFP with high IR then I would not use it.

 

[Wattson]

Yeah, that’s wild—147 mΩ for a LiFePO4 cell after just two years and under 100 cycles? For LiFePO4, IR creeping up that high usually means the cell’s degraded quite a bit—could be from over-discharge, heat, or just a dud from the start. I bought a batch of new batteries and prepared to assemble them. I tested the IR and it was 0.25mΩ on average. If necessary, I can recommend the product to you.