The BMS flickering and saying the cell is good then bad, could be because it's adding voltage, but then because the cell is faulty, suddenly the voltage is fine (microscopic capacity), but then as soon as it stops adding voltage, the voltage falls back to zero and it has to turn on again.
This could be a faulty cell, or, it could be a faulty BMS that killed the cell. It's hard to know.
What I would've done, first thing when I got the batteries was to make a list and measure each cell's voltage. That would rule out cells that were suspiciously low.
Then put some small load on the module/pack for a while, incandescent light bulb or whatnot, just to give it "some" drain, then test the voltages again after a while. That would rule out cells that appear to have the correct voltage, but only because their capacity is practically zero.
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The part where you showed good thermal conductivity... it's hard to tell over camera, but to me that looked like you had quite poor thermal conductivity and quite a bit of thermal paste. By the time you clamp them together there should be almost no paste between the two layers of metal. It should be half-transparent when you pull them apart again. Again, the metal on both sides should be touching, the thermal paste just microscopically fills in the places where it is not. It might still be fine, you'll of course only find out under hardest use whether it's sufficient (especially because you have no better examples to compare it to), so, maybe much ado about nothing. You're building a supercar, so, little details might matter to you in the end, performance-wise. And, performance-wise, heat does become a limit.
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Discouraging week for you it seems. Especially when you pay for things, you'd kind of expect them to be working (and tested) properly. You're not bodging together junk you found and expecting it to take some extra labor to figure that out. Hopefully EV West straightens things out for you. At least they're more reputable than the kit company
