On the topic of cell swelling...
It turns out that there is an optimal pressure/force on the sides of a
Li-Ion pouch cell. Typically, it is ~12 psi over the entire side. (This
will vary of course depending on the specific chemistry, brand and style
of cell.)
If your cell clamping system exerts much more or much less pressure, the
cycle life will decrease significantly. The specific power will also
decrease if the clamping pressure is too low.
The difficulty is that the cell volume increases and decreases about 1%
to 2% when the cell is cycled from 100% SOC to 0% SOC. The clamping
system should allow this small expansion while still maintaining the
pressure are near the same ~12 PSI. Also, the cell volume increases ~5%
(or more) as it ages, which makes matters even worse.
You basically can't constrain the pack/module rigidly or the pressure
will shoot up way out of the optimal range quite quickly. The constraint
system must have a bit of spring to it to maintain the target pressure.
Quite tricky to hold constant pressure. Years ago, I figured out how to
do this and we wrote up a patent. In a nutshell, you carefully size the
straps that hold the pressure on the module to plastic ally yield when
the pack pressure gets too high. When you charge and discharge, the
straps stay within their elastic range and thus expand and contract
normally, thus holding the pressure in the correct range. Sort of like
setting the drag on your fishing reel when you have a big fish on.
https://patents.google.com/patent/US20110177377A1/en
I made a few bucks on this patent, but Boulder Electric Vehicle
actually paid a handsome sum for the patent attorneys so they collected
the majority of the benefit. No doubt, high-end Li-Ion packs are built
this way these days. If the constraint straps (or bolts) have a "dog
bone" shape with a narrowed middle section, this is the reason why. :-)
This thin section stretches, just a bit, as the pack ages.
Bill D.
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