Most of the time when battery packs go bad, it is only one cell. This means that you can get the cells for virtually nothing by just collecting old laptop and power tool battery packs and just keep the good cells. I thought about doing it since there are many good You Tube videos on how to do it, I just have way too much else going on. The only “exotic” tool that you really need is the welder that is used to connect the nickle strips to the batteries but, I bet even those are cheap now. It would be a great side business for someone.