I use a couple of those drills, including a 7.2 volt Makita, all the time
directly off the vehicle battery. They are very tough, you just get more
torque and rpm. I was actually just curious at first, figuring I'd just
throw the drill away if it burned up. I use them all the time though and
they continue to work fine. If you want to drop the voltage the diodes in
line are the cheapest and best way to go. Resistors are no good for when
current increases due to motor load demand, voltage distrubition to the
motor will be decreased and there will be an overall loss of power
available at the motor. My advise is to just hook it up to the 12 volts
and don't worry about it.