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.