Interesting point. I don't have the LF280 datasheet, but in the newest LF280K sheet, version A 2022.04.20, that Amy Wan posted in August it says the following:
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and then on compression force it says:
View attachment 113062
Interesting. I saw the 280K datasheet when it first came out (and added the data about increased lifetime with a 300kgf fixture) but there was no data on expansion force in that early version.
So, these have up to 50 kN expansion force at EOL, and compression force cannot exceed 50 kN. I'd read this as you just barely should be able to stack the cells without causing internal defects in other cells from one going into EOL expansion.
Again, knowing what the expansion force at HOL (half of life) sure would be helpful, but my general sense is that it’s going to be practically impossible to design a rigid fixture that remains rigid under 5000kgf(!).
So I take this new datasheet to mean that enclosing these cells in a rigid metal box such as that used for an EV battery will be fine.
But note that this is how I read the data, and I'm in this thread to try to clarify my reading, and this is my first endeavor to build a LFP battery.
If I had seen this datasheet before building my 300kgf clamping fixture, I would not have bothered.
On the other hand, if you are not using a ready-made box for the purpose and are already going the route of rigid end plates secured by threaded rod, the additional cost and effort to add calibrated springs to apply 300kgf is modest.
I don’t see anything in this latest datasheet specification to suggest that lifetime increases if compression force of greater than 300kgf is applied (though the risk of causing damage by applying even 3000kgf would appear to be low).
Yes, I haven't seen any data like that either. Nor how much one could expect the cell to expand with a given force.
Unconstrained 280Ah cells expand ~1mm in the center of the cell (from a starting point that might be sunken with respect to the rigid top and bottom caps).
My approach was to ‘calibrate’ my 300kgf clamping fixture when my cells were at their lowest usable SOC. Calibrated springs allow your to translate force to a number of turns of a nut on a threaded rod, so X turns on each nut from first compression translates to 300kgf.
My springs were chosen to maintain close to 300kgf over a compression range of 1mm-per-cell (16mm for the full pack) but my first charge to maximum SOC expanded by a fraction of that (possibly 2-3mm, possibly nothing at all).
If I’d seen this data and wanted to go to the trouble of building a clamping fixture, I would have had no concerns using a shorter calibrated spring (though the only advantage would have been slightly shorter overall dimension, since spring cost would have been about the same…).
My plan is to automatically monitor the spring deflection, but that's more of another layer of safety by checking if anything starts to deviate. And since my battery is for a UPS it won't be cycled enough to give any data in the nearest decades.
Been there, thought about doing that, decided it wasn’t worth the trouble and wouldn’t even consider it after one year of experience with my battery now.
Getting SCC and parameters set up to correctly charge and discharge cells within target voltage ange is a much bigger concern to me than anything to do with expansion or clamping force at this stage.
I got pretty lucid with cell matching but have a ‘runner’ (cell with lower capacity than the others) so making sure that cell is the one limiting capacity (meaning both the first to reach lowest SOC as well as the first to reach maximum SOC is my greatest concern.
The BNS is there as a fall-back safety device but when the BMS cut-off kicks in, it causes a fault to the overall system requiring a manual reset.
So my biggest concern now is keeping an eye on that weakest cell to assure I’m staying one step ahead of it (reducing usable capacity faster than mismatch increases).
I have no clue about what additional monitoring exists in cars using rigid fixtures. They might just disable any use of the battery long before reaching EVE EOL. Thus I feel that I can't just adapt that practice and point to others doing it.
I doubt any EV battery out there will not deform under 5000 kg of force. So first, batteries are unlikely to be used for anywhere close to the service life needed to generate 5000kgf of expansion force, and second, even if they did, the metal box is unlikely not to deform a bit and hence apply less than 5000kgf.
In addition, even if they do get exposed to 5000kgf, worst-case would be formation of some ‘internal defects.’
‘Leakage’ does not occur until over 100,000 kgf (before which I suspect the battery box would burst).