Compression of Li-ion cells is good - but not too much

View attachment 226 Effect of compression on Li Ion cells.pdf

Compression of a stack of electrodes in a Li-ion cell results in improved capacity retention, but at some point, it's not as helpful since it retards ion movement within the electrolyte. About 1.3 MPa looks to be ideal.

Interesting. I had to convert the pressure units to get a better feel. 1.3MPa = 189 psi. That's quite a bit of pressure.

When I was reading about lithium pouches that were used for racing, I recall that having the flat sides compressed improved the peak amps, and also reduced puffing. They knew they were beating on the cells, but they wanted to find out how light they could make the pack, while still drawing "X" amps.

Once that was determined, the packs were considered single-use after a break-in period of cycles on the bench. Race it, then toss it.

They also warmed the packs, I think it was around 100F, once running, they had active cooling to prevent the cells from exceeding 140F

We're all sort of wondering if that shouldn't be kilopascals - it equates to one ton of pressure on a 135 mm x 50 mm pouch cell!

I'm curious about the units, too. My current pack is built up from prismatic e-car cells, and to achieve its design cell spacing it requires enough compression to visibly bow the 1/4" thick aluminum endplates. I don't think it's anywhere close to 189 psi. However, it's much closer to that than to 0.189 psi.

The pressure topic has come up before:
https://endless-sphere.com/forums/viewtopic.php?f=14&t=52244
in regards to A123 pouch cells, if it helps.

IIRC, Wb9k from A123 Systems said something less than a few PSI was what they specified for their cells. But I couldn't find a specific number.
https://endless-sphere.com/forums/viewtopic.php?f=14&t=52244&p=799311&hilit=psi#p799311

EIG's factory-created hardware does not directly apply compression to their NMC pouch cells, but simply prevents them from expanding, except for the end cells. (one of which is under a plastic sheet cover that's stickied onto the end frame, and one of which is under a non-fixed thin aluminum divider/heatsink cover plate). I don't remember if I've seen a spec for compression pressure on their cells. (but the only one I've ever had swell up was reversed by overdischarge of my lighting pack some time back).

I don't know about any of the other factory pack hardware setups.

The Nissan Leaf modules, whilst assembled together using metal housings , bolted together,.... were reported to have no direct pressure on the actual cell pouch's, which have clearance space inside the modules.
The edges of the modules take all the assembly bolt force.
.....but then,... Some of those packs do not have a great record for long life. !