I harvested dozens of laptop battery packs (100s of cells) when I built my first e-bike battery from them. Tested capacities, sorted and repackaged into a 70 cell pack (5px14s 52 volt) for the bike. Most laptop packs had one or two cells that failed while the rest remained good. You will need to test and sort cells by capacity so the BMS isn't always trying to balance the odd cell which can be a problem for heat/fire. If you go this route you'll need some
flush cut snippers to cut off the old connecting strips. Be sure your soldering skills are up to date (clean, flux, tin, solder) or you have a
spot welder for battery cells (they are not all created equal).
I have never used laptop cells again as the pack failed after the first year with electrolyte leakage (see below) from mismatched and failed cells. (The brown residue is flux that I forgot to clean off. The clear gel is the electrolyte.) I have made several packs since but always use matched cells that are the same (most recently Tesla
Model S cells).
I have quite a bit of experience in cells/packs/BMSs and I would never repair one for a customer. The risk is too great for the meager rewards.
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