12 v is12 v you wont hurt anything , amperage is power used and
capacity. If it is 12v nicad . 12v nicads are dead at 12v, fully charged
at 13.5 per cell. So if it is a 10 cell 1.2v pack 12v is dead.and drill
will be slow at 12v. Fully charged car batteries are apx 13.4v, so keep
battry charged and it will work fine. The original battery operated
drill had a separate pack, A fuse wont be necessary except for shorted
wiring. You could probably run it at 16 v or more. and not hurt
anything.