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?

Well, that depends how many amps the blanket will draw. In colder weather, I think it's harder for the battery to work at its maximum efficiency, so I'd be tempted to err on the safe side and run the car longer than normal. I mean to say, I'd be less likely to use the blanket while the engine was off.

The wife has one of these blankets. Its a bit larger than you are describing. The thing is awesome but it takes a bit of time to reach a good level of heat. I actually saw some small heater fans that are designed to be plugged into a cig lighter of a car. They blow a surprising amount of hot air, but seem to burn up a lot of power. At my fav outdoor store, they actually sell electric socks, which are battery powered believe it or not. I got a pair for bow hunting for those times when you have to remain in place for long periods. These things are the warmest socks I have ever worn in winter. I wonder if they make battery operated blankets as well??? Like you said, if you run the engine for a while to warm the blanket, even every second or third time you use it, then you will spare the battery for when you need it most.

12-volt electric blanket
-Blast

I think I might be tempted to take a good car battery, on its own, and see how long it'll run the blanket before it dies. That would at least give an idea of what to expect.

You could, but I'm not sure that's a good idea. As I understand it, a standard car battery can suffer permanent damage, or have its service life reduced, by a severe discharge. Most inverters cut out at around 10.6 volts DC to prevent this.

Why not just carry a full size duvet? You are in a car; youv'e room.
The Sock

I'm with you on this one. I would carry adequate warmies that are non electrical. I wouldn't want to depend on a car battery for something as vital as warmth - keep it for starting the car.

I also agree. Using an electric blanket assumes that you will have the vehicle and power. If you have to leave the vehicle for any reason, the electric blanket isn’t going to help you much.

I would only recommend the idea if you equip the vehicle with duel batteries. This is a standard set up many 4 wheelers use. High draw and most acessories draw from the second battery. Then rvrn if the battery gets too low the primary battery is used to start the vehicle which in turn recharges both.