Who's got the oldest cells & how are they doing?

[svetz]

I've been reading that some LFP cells have organic electrolytes that break down over time, and some use special coatings on the anode that swell over time, and that they'll only last a decade. It was even recently posted that they're unsafe once they fall below 80% capacity.

But, I've also seen papers regarding repurposing packs from cars after their end of life in that venue as home and grid storage. In fact, I believe @GXMnow is one of the folks that's done this with a chevy bolt pack and keeps an eye on the "fire" news related to it.

So, figured the folks in this forum know what's really what. Here's some questions, but I don't know enough to know if I'm asking the right things...

• Is it that quality cells will work and "cheap" cells are dangerous?
• Is it confusion between lithium-ion and Lithium iron phosphate chemistries?

If your cells are older than a decade, what percent capacity are they are and how far will you run them before retiring them? Or are there specific things you annually inspect each cell for to decide to replace it (e.g., internal resistance, rust, swelling)?

 

[sunshine_eggo]

So, figured the folks in this forum know what's really what. Here's some questions, but I don't know enough to know if I'm asking the right things...

• Is it that quality cells will work and "cheap" cells are dangerous?
• Is it confusion between lithium-ion and Lithium iron phosphate chemistries?

At least in the U.S., most PHEV and EV applications are NCA, NCM or NCM-LMO.

 

[upnorthandpersonal]

It was even recently posted that they're unsafe once they fall below 80% capacity.

This stems from the idea that dendrites form with aging and can cause an internal short. Dendrite formation depends a lot on how a battery is used. In addition, catastrophic failure is mostly related to non-LFP chemistries which exhibit thermal runaway.

 

[svetz]

That's why I'm asking, interested in what the 2nd life builders are doing and if they've found a particular measure they're comfortable with. For example, do they retire a cell when the internal resistance is 1.5x the original or the swelling is >2mm? I did find some measurements (see right image), the image is also a link to the source.

 

[toms]

My oldest LiFePO4 pack has turned ten years old. The pack has never been outside the range of 3V - 3.5V, and is a 400ah pack that i only ever use 300ah of.

I still get 300ah out of it, the voltage sag under load is the same as when it was new.

There is a massive difference in the different types of LiFePO4 chemistries and constructions, it is a common fallacy that all LiFePO4 cells are similar.

I keep in contact with a few people from the time i was purchasing my cells, many have batteries older than mine that are still going strong.

 

[jberger]

That attached Cell Press article is a great overview, but keep in mind that it's about Li-ion not LFE.
I often find myself going back and re-thinking the "conventional" wisdom since most of it is actually based on Li-Ion not LFE.

In this case, I don't think there is a high risk of a reduced capacity LFE cell bursting into flame, but there is a real chance that a "2nd life" cell will begin to behave erratically as it's capacity falls below 80%. From charging and balance to the potential of a shorted cell, the failure modes will increase.
I think the bigger concern is how you would need to reconfigure a pack over time as you loose cells, especially the 2nd life cells since the IR will be so different from what you might find as a replacement.