when the batteries go out, buy a new rig. Have you priced batteries recently, you will find out that one battery is almost half the price of the intial rig.
Consider trading up to a porter cable rig. I have an old 12 volt that is pushing 5 years old and I use mine almost daily.
Just a few comments on hp on these drivers. I found that a 12 is great, a 14 is still good, but the higher V's I have tried even when torquing down, will pop heads pretty easily and almost too much for these cheaper drywalls crews.