Some start confusion to make..
You seem to assume the surface is FLAT.
Probably you did not receive your cells yet, or didn't look at them in this way.
When you receive they are )(
During charge the expansion starts at the center and 0 on the outer edges.
You probably have a 5x5 cm surface that takes the "first hit".
2 cells expanding, at the center on a 5x5 cm surface.
When they "hit" touching eachother), the expansion force is gradually expanding over larger area.
"Never" the full cell surface, about 15mm from the edges the 90 degree angle of the aluminium sheet will keep it in nice shape.
So what's it all about??
Without clamping the "bulging" at the center have little to none force to prevent delamination of the sheets.
You don't want this.
The 12psi ??
In blocks of 8 cells, Eve uses 12PSI for their laser welded aluminium box to compress the cells together.
Each cell with a thin separator sheet (I think plastic)
Not 8 X 12psi..
The outer edges of the cell first will take all the force.
The cells are still )( shaped.
After many cycles...
They will try to bulge, but can't.
Metal box prevent this from happening.
How much force is there between each cell, and if it is equal on all sides?
People a lot smarter then me can try to calculate.
With the variable that they don't know how much the expansion and the "urge" to "bulging" is.
What we do know.
Snug fit, some clamping is enough.
They won't expand.
And if you, after a few years, are dumb enough to unclamp them, take then out the snug fit.. (probably need to destroy that box) it will never have the same size again.
Sure.. you can clamp it together.
And it will work for a while.
My personal experience in this, is self discharged cell, aka short, aka fire.. and that one cell took down $2500,- of lifepo4 batteries..
Better don't.
Don't try to fix a winning team.
For me, a bit of bad luck with 152Ah cells who had the same housing as 120Ah..
Somehow.. that 32 AH extra let the cells expand quickly (days/weeks) ..
Not really somehow... Engineering mistake..
Anyway.. I tried to get them more flat again, not on purpose but by clamping, the delamination seemed to reduce, and by hand I could tighten the nuts from the 6 bolts.
After a few months...
Spontaneous fire...
You can not uncook an egg, or delaminate cells.
You can prevent it, and all it needs is a little compression at the start when you build your battery pack.
The 12psi is an other word for snug fit.
Prevent expansion.
How much PSI is generated by the cells...
That can be a lot!!
After the fire I disasambled the battery pack.
Yes, many hot way to hot and look like balloon.
The clamping prevented this from expanding.
I removed (or tired to) the 6 X 1/4 inch threaded rods I used for the clamping.
With 2 left, the one 1/4 inch rod snapped in 2 when I tried to untighten the nut.
People smarter then me can calculate how much PSI was on those 2 rods before it will snap.
I keep it simple : too damn much!!
In a normal situation, it should never be this high.
I was genuinely surprised that the vent didn't rupture!
More simple..
The cell wants to expand.
When you prevent this, it can't expand.
But will generate force on the material you used to prevent it.
When you can keep your battery pack upside down without any cell sliding out.. it's a strong enough (snug) fit
Any more force is useless.
The outer edges of the cells take that force and has nothing to do (yet) with expansion.