[Wile E Coyote]

Assuming you can get the battery connected properly to the drill, df hit on the main problem. The battery would be pushing higher-than-rated voltage through the motor during any high demand times. I think it would be easy to kill the motor if'n you didn't pay extra special attention. That's just me bench racing, though. I've never done it. I have however, seen how quickly headlights fry during unregulated alternator output. (hint: in about the time it took to read that sentence)