Constant-Force Lithium Cells Clamping

Designing this kind of clamp, while allowing for cell expansion, does not have to be complicated.

Indeed, foam materials for EV packs have been developed to do just that. Like Norseal / Poron

This type of foam has predictable compressive force characteristics, and remains elastic even after prolonged compression times. Available on McMaster Carr as a 1/4" thick 3' x 4.5' sheet for $125, sku= 86375K64. More than enough for our 2 packs. It provides about 11PSI of pressure (i.e. ~300kg total over the LF280's broad side surface area) when compressed 25%.

Here is an example of Poron's compression force deflection characteristics. It is likely not the same Poron sheet as MacMaster's, but seems in the ballpark:

Studies on life-versus-pressure for Lithium cells show that pressure between 6 to 18PSI, i.e. ~150kg to ~450kg force for LF280, provides most of the longevity benefits:

Finally, EVE Engineers have supposedly reported that going much beyond 18 PSI has a detrimental impact on cell life.

Ok, so now that we have all the data, how thick a sheet of foam is needed ? A 1/8" thick (3.2mm) Poron pad would likely be too thin to guarantee a safe constant-ish pressure over the cell's 1.2mm expansion (though a cell will likely expand less than that once compressed).

On the other hand, 1.2mm is only 19% of a 1/4" thick sheet. That is well within the linear portion of the compression graph above. Sold, 1/4" it is, validating the McMaster item above 😎

Conclusion: the plan is to initially clamp the cell stack, fully charged, until the foam is ~35-40% compressed. That should result into a ~350kg compressive force. When the cells are empty, the foam pads will now likely be only ~15-20% compressed, yielding ~200kg.

So, even in the discharged state the cells will be very firmly held in place, meeting the safety goals previously defined.

Cherry on the baklava, the foam pads will also provide electrical & mechanical separation between a cell's case and the surrounding cells / clamp / enclosure. Which is a good thing in mobile applications experiencing harsh vibrations. The cells' thin blue vinyl wrap is not designed to withstand sustained abrasion.

Note that the pads will add a thermal insulation layer between the cells, potentially leading to large temp imbalances. That is a no-no for the EV uses that LF280 is designed for, which demand very high 2C+ currents and duty cycles. However, in this van's use case, the very mild and intermittent currents will not lead to much thermal stress. I'm more worried about keeping the cells warm in -4°F / -20°C weather...

Sweet, so we got 3 birds (expansion, longevity, separation) with just 1 clamp setup !