I have an "old" 12 volt Makita. After about 15 years of intermittent use
one of the batteries started to short the charger. The local BORG wanted
almost $50 for a new one and some of the E-tailers wanted about $30 plus
shipping. One day I was at HF and saw a replacement 12v for their grease
gun. It looked like it just might fit. Th insert portion was right but
it would not quite go in far enough. I could have carved away some of
the drill handle and made it work, which I didn't want to do. After some
study [the HF unit was put together with screws] I decided to chisel the
old battery apart. With a little judicious moving and re-soldering of
just the two top cells I now have a new battery for $12 and my time.
I didn't bother to re-glue it as the clip that holds it in the drill
also holds it together.

Chuck P.