Is the FFE Battery a Time Bomb?

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27ace27

Active member
Joined
Jun 25, 2023
Messages
35
Howdy All,

I've done a lot of research in my efforts to find a viable fix for the coolant leak that will eventually kill all FFEs made before 2017. I've learned a lot, but it all got me wondering what a hard EOL would look like for the batteries in our cars. Would it be catastrophic? As best I can tell: probably.

All lithium pouch cells made with current technology will swell with cycles and calendar aging. However, as far as I can find, there have been no experiments done to see whether this stops at some point. And as EVs are still fairly young, there's no real field data either. If it doesn't, the ultimate fate for all lithium pouch cells is rupture. Given that the gasses which cause the swelling are flammable, and most people charge in their garage, this could easily be catastrophic.

The FFE batteries are both good and bad here. The good is that they have active cooling, which significantly slows down the rate of gas production. Temperature is a huge factor in how quickly gas is produced. The bad is that the heat exchangers which do the cooling are rather fragile. Specifically, the aluminum is so thin that it does not take much pressure to crush the fluid passages. Indeed, many of the heat exchangers I removed from the upper battery of my 2012 FFE were severely crushed.

In this way, the FFE's battery cooling system may do more harm than good in the long run, even in the 33.5 kWh packs. The tiny differences between cells at manufacturing will mean significantly different rates of swelling 10+ years down the line (think butterfly effect). As a result, the heat exchangers will crush at different rates. Since a crushed heat exchanger cools the adjacent cells significantly less, this will inevitably cause uneven cooling within the cell stack. And since there is not a temperature sensor on every cell, it is very possible that some cells could even overheat. If nothing else, the uneven cooling will cause the hotter cells to swell faster.

If there is indeed no limit on cell gassing, this would cause a positive feedback loop. Temperature uniformity would decrease and stack pressure would increase exponentially. The endpoint then would be either thermal runaway of an overheated cell, or rupture of one or more cells. In the case of the former, the band-aid fix of removing the heat exchangers entirely would only make a catastrophic outcome more likely. Those heat exchangers provide a thermal barrier between cells, reducing the likelihood that a thermal runaway event in one cell will propagate to its neighbors.

A rupture would reduce stack pressure of course, but now the cell's volatile innards are exposed to the atmosphere; specifically all the moisture & oxygen therein. I can't find any definitive evidence for lithium cells, but NiMh cells at least are liable to spontaneously combust when exposed to the atmosphere; even when fully discharged. Though I have seen an image of a burst cell from a Nissan LEAF battery that didn't self-ignite.

What do y'all think? Is catastrophic failure inevitable for all lithium cells or is there something I missed?
 
I have a feeling that the usable capacity will have degraded to an unbearable point before it's likely to rupture. Keep in mind that the end of useful life for batteries is typically 80% of the original capacity. Any info on the point at which outgassing ramps up?
 
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