I finally remembered what I was going to ask about earlier.

Most of us have inverters in our cars, so we should be able to power a small electric blanket or large hot pad. Which one is more efficient:

-running the car every so often to recharge the battery a little bit, then turn it off and stay with the battery?
-running the car every so often to reheat an entire cabin full of air?

I'm not talking anythign huge, just like a crib warmer or something. Put it under you in the car seat, with a blanket over you. Turn the electric on for say 15 minutes every hour. Or will it draw too much charge off the battery?