If it's getting that cold just remember that Lithium batteries are not built for low temperature operation.
Low temperature operation
Chemical reaction rates decrease in line with temperature. (Arrhenius Law) The effect of reducing the operating temperature is to reduce rate at which the active chemicals in the cell are transformed. This translates to a reduction in the current carrying capacity of the cell both for charging and discharging. In other words its power handling capacity is reduced. Details of this process are given in the section on Charging Rates
Futhermore, at low temperatures, the reduced reaction rate (and perhaps contraction of the electrode materials) slows down, and makes makes more difficult, the insertion of the Lithium ions into the intercallation spaces. As with over-voltage operation, when the electrodes can not accomodate the current flow, the result is reduced power and Lithium plating of the anode with irreversible capacity loss.