An apparatus and method employ a pair of input terminals to receive an input voltage from a charging power source for charging a backup power supply for an emergency lighting system, the input voltage corresponding to any one of a plurality of nominal operating voltages. A voltage level detector produces, in response to the received input voltage, an input voltage level signal indicating which of the nominal operating voltages is being received. A temperature detector determines whether the temperature of the backup power supply exceeds a threshold and produces a temperature signal in response thereto. A heater includes a plurality of heating elements, and is activated to heat the backup power supply when the temperature of the backup power supply is less than the threshold. The input voltage level signal determines which of the heating elements is activated.