A process of controlling the temperature of a battery pack includes the steps of determining the operating mode and present temperature of the battery pack. Optimal temperature for the battery pack depends on the operating mode and the difference between the present temperature and the previously identified optimal temperature. The battery pack is warmed if the temperature difference (measured minus optimal) is large. The optimal time interval over which the battery pack should be warmed is a function of the operating mode and the previously calculated temperature difference. A heater is switchably operated enabling and disabling the heat generating element to warm the pack to the previously identified optimal temperature.