that's what you're NOT supposed to do. it says it all over the manual. ni cad batteries will get a memory to them if you do that. then they'll never take a charge again. same thing with a car battery. once or twice is usually ok but after that it it will never charge. it's called deep cycling. that's why you see some car batteries that say "deep cycle" on them. it means they can be ran down really far a lot of times. the manual for my stuff says, as soon as you notice degrading performance to stick them on the charger. it also says to leave it on the charger when they're new for 11 hours. I seen a few places on the net say craftsman replacement batteries are "recondidtioned" old batteries. a lot of people are complaining about them. I bought my 19.2V set back in 2004. everything was used almost daily all day long. especially the drill. and the stock batteries have been replaced 3 times. it's actually cheaper to buy another drill just for the batteries. because you get the drill, charger, 2 batteries and a case for $89-99 usually. 2 batteries is $60. and that's how I ended up with 3 drills, 2 chargers, 5 batteries and all the other tools.