Well......unless you're automotive battery is hooked to a typical charging
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. These could be nearly
anything with a rating of say 10 amps @ 50prv min and wouldn't have to be
even the same........use what ya have. This solution will have the added
benefit of giving you reverse polarity protection should your cable to the
auto battery get reversed. If you bring your cord into the bottom of the
old battery compartment there'd be enough room to include the string right
in there as it's quite long....strain relief below that. Realize that
your auto battery voltage will sink as you use it (uncharged....portable so
you can lug it close to where you want) and this won't hurt things at all.

Let me know how ya make out (have a couple of these drills myself). I can
picture powering it off of one of those booster battery packs as a good
solution for portability..........use a cigarette lighter plug so you can
power right off this or your vehicle or via a battery clip adapter for open
other types of batterys.


Gord