A method for heating a battery in a hybrid electric vehicle. The hybrid electric vehicle has an engine, a battery, a motor generator powered by the engine or the battery, and a control module. The method includes the steps of determining a battery temperature and determining whether a tip-in event, a tip-out event, or a terminal voltage event has occurred. The polarity of the battery is reversed if the battery temperature is below a predetermined value and if a tip-in event, a tip-out event, or a terminal voltage event has occurred.