I was reading an article about making battery packs for model planes, and the fellow was using a “Pencil” spot welder to weld on the tabs for the battery cells.
If I ever jump to cordless tools, that might be a good addition to allow me to remake my own battery packs.
I gave up trying to install replacement batteries. I just recycle them now. Makita in particular was fussy as hell. Someplace I read that they have a chip hidden in the battery pack that keeps track of battery use. Sort of like Canon or HP printer ink. But I have no real knowledge of it. Just hearsay.