Since you’re not a pro, and you say those are old batteries, I’d reccomend just upgrading to a new battery because as batteries age, especially if you’re in a warm environment, they tend to lose both their charge, and their ability to hold a charge well. One way to slow down this chemical breakdown is to store your batteries in the freezer, if at all possible. Although, I’m not sure if that would work on rechargable batteries as well as it does on regular , nonrechargable ones…I hope this helps. By the way, welcome to Fluther!