12v to 18v for tools from car battery
I think you would be better off with a DC-to-AC inverter to charge the battery packs for cordless tools, or use corded tools with the inverter directly.  That is a lot of power so you would probably need a number of 12V batteries depending on how much work you are going to do.  Also note that the power levels you are working with can be dangerous and all the components need to be sized properly.