Run a 9.6 volt Makita drill off 12 volt car battery
system, your auto battery at rest will be more like 12.3 to 12.6 volts.....current will only be in the range of a few amperes but will be quite variable as pointed out. I''d try a string of 4 silicon diodes inline with your made up cord for a typical voltage drop of 2 volts which will be somewhat constant despite the current drawn.