Building Charger To Fix "Bad" LiOn Batteries

My friend has several of these Ryobi 40 volt batteries and I have a couple of them myself that say they're "bad". I've taken them apart and examine them and I don't think they're bad, so I got a pretty hefty naked charge booster that will take anything from 5 volts on up and is capable of charging up to 50 or 60 volts... So now I can get these ryobi's fixed.