Yea, I had thought of that too. So I guess when I get in and start the car, the 12v would be more topped up, and would not require as much juice from the big battery. I guess it couldn't hurt to use it, but Someone mentioned that it might shorten the life of the 12v to have it constantly trickled when not in use. I really don't know about that, but I wouldn't think that it could be shortened by any substantial amount. But again, I don't know for sure.