A built-in battery header would serve as a workaround in many circumstances, and is already being worked on.[0] The heater would draw power when the car is charging to either keep the electrolyte above room temperature or preheat prior to expected use. You could even run the heater off standard 110v or 220v outlets, for instances where the car is parked but doesn't need to actually be charged (a garage might not be willing to deal with EV charging, but be fine with the relatively minor draw of a car's built-in battery heater in comparison). And when that doesn't work, you could probably run the heaters off the battery if you keep it from dropping too far, or a secondary battery as suggested by the grandparent.

There aren't really any perfect solutions, but cold temperatures are a problem we've dealt from the beginning with both batteries and internal combustion engines. My dad used to tell me stories about his time in Alaska as a teenager, and a bush pilot who would talk about when he'd have to pull the drain the oil from his plane, pull the battery, and take both inside overnight to keep them from freezing.

0. http://popularmechanics.com/cars/hybrid-electric/a21999984/e...