bit late but i was looking for something along the same lines, unfortunately after looking, nothing really beats propane for the blistering cold. i would highly recommend checking out the Mr. Heater Little Buddy
Propane Heater i read about here.
its compact, portable and easy to use. even has some built in oxygen sensors and tip over switch that will auto shutoff in times of danger as far as i know. if you choose to give up the search for battery powered, check this one out for sure.
hopefully those searching the forums can see this if they have the same question