All right, it is going to be pretty cold this week (highs in single digits and below zero for one day). Unfortunately, school starts back up after X-mas break, so I won't be able to plug my truck in in the school parking lot. It has started below zero with only the grid heater, but I want to make it easy on it. What if I hooked up an outlet to a deep cycle battery, to a power inverter, and plugged my block heater into that? I would put a timer in there, so It kicked in an hour and a half before I start it. How long do you think the battery (fully charged) could support the block heater?