I was reading on another forum about converting 12V battery tools to run off your vehicle battery for off-road repairs etc. This sounded too cool so of course I made one right away. I have an old 14.4V Makita drill that I thought about converting which would work great with the engine running. But then I ran across this 12V Porter Cable 2 piece impact and drill set that is on clearance for $70. The impact driver is pretty light weight but for trail repair and smaller stuff it should suffice. I bought a light weight set of jumper cables for the battery connection from Walmart for cheap. I took one of the battery packs apart and removed one of the battery cells to make room for the cable, drilled a hole through the bottom and soldered the cable in place of the lithium battery connections. I did leave the other two cells in the pack for weight and feel purposes. Works like a charm and since I just modified the battery pack itself I can power any 12V Porter Cable tools with it. They even have a reciprocating saw and since they've gone to either 18 or 20V the 12V stuff is cheap.
12V Porter Cable