Its a bad idea to use your main vehicle battery for WFP, as you put a lot of strain on it and may not be able to start the engine after you've finished the job! A 2nd battery (lots of ppl say a leisure battery, but a car battery will work too, even if it won't last as long) and a split charge relay is the way to go.
Its not a big expense, but could save you a lot of hassle!