You can restore car batteries and even golf carts you can keep them going for years. Keep your cordless drills and other tools going on their original battery for years before the battery gives up. If you have bought replacement batteries for your drill you know the cost can be crazy.
Again, obtaining used batteries from other sources for free or very cheap can give you an unlimited supply to work with, click here to learn how to test batteries to see which have the best potential for reconditioning. You will be saving some big bucks and even have a chance to make an income doing this.