Ah, some concerns, yes.
Yes, I'm painfully aware of the magnitude, but the price difference is worth it to me. I can do this stuff at night and during weekends, when I can't work to earn money to just buy new stuff..
I have a selection process in mind though, and building an automatic discharger that can do a bunch of individual cells at the same time shouldn't be a problem.
Not too worried about dead cells, if they take a charge, and don't self-discharge, I'll test their capacity and see from there. Some dud cells are to be expected, especially in the batch of "bad" packs, but less in the batch of "used, pulled working".
I might include a tile and a bag of sand in my (dis-)charging station though, if one catches fire for whatever random reason.
The ones that stay dead, will be brought to the battery recycling station, acceptable losses.
For now, I have 165 laptop batteries (mostly 6 cell?) on the way, and some sample cells from the 3000 cell batch. I'll see how they behave before I buy an even larger quantity.
Fusing the cells individually was the plan already (see first post).
I know these aren't the best cells for high power applications, but at $0.25 each though, instead of $2.20. I'm okay with a little worse specs.
I'll be using active ventilation and temperature probes all over the pack. Don't want it to blow up and set my car on fire.
I might need some other type pack in parallel, to put out enough power for acceleration and to help store regen energy, I don't think these cells like doing much above 1C discharge rate. Perhaps I'll use a Prius pack, who knows.