@Panglossian Never, ever let batteries sit outside in the cold as that will zap them. My winter rides are done in the -2C to +4C range, no problems. But if I leave the board in the car at those temps (the typical "oh, nice weather, gonna cruise after work!" scenario) then the battery will be dead. So bring your board inside and store it somewhere warm to keep the battery charge intact, expose it to the cold when you want to ride and the battery can keep somewhat warm due to discharge.
@HanahsDax Nice info :)
I got an error code 15 when I forgot my board in the car for 6h at around 0C, it confused the hell out of me at the time until I realized the cold had drained the battery.