My concern would be how long would the battery keep the heat going. I would not want to run the battery down low and not be able to start the car. While better than running the engine to run the car heater, I think you would need to monitor the battery charge to be sure you don't end up with a dead battery and no way to start the engine to charge the battery for more heat. Maybe having one of those portable power units that claim to be able to "jump" your battery is a possibility. I also understand there are retrofits for batteries that have a switch and sensor that cuts the battery out when the voltage gets low, but can still start the car after resetting the switch.