One of the things that can go wrong with rechargeable batteries is that as
the metal plates out upon recharging it tends to form, well , stalagmites
(for lack of a better term).  Eventually, these stalagmites can bridge two
plates, rendering that cell useless.

Prevention: Completely discharge the battery before recharge (can be a real
drag and really inconvenient).

Cure (Theoretical):  Get a really big capacitor, like the kind you might
have found in a Who tour amp, charge it up, then discharge it to the
battery.  I say "theoretical" because we tried to do this in my high school
electronics class, with very poor results.  This doesn't mean it won't
work; the battery/capacitor we were using might have been defective for
other reasons, or whatever.

Good luck,
Eric.

PS.  You could always find the Voltage/amp specs for the motor in your
drill and make your own battery setup.