DHC 8 First Officer experiences an AUX BATT HOT warning light shortly after takeoff. The battery is turned off and the flight returns to departure airport. During the return the battery temperature continues to rise and the crew declares an emergency. ARFF follows the aircraft to the gate and are instructed to disconnect the AUX battery.

Narrative

At approximately 400 FT AGL; the AUX BATT HOT warning light illuminated along with the master warning light. The pilot not flying and myself confirmed the aux battery overheat by the battery temperature indicator; and turned off the aux battery switch. Battery temperature was initially 64 degrees C. We continued to complete the after takeoff checklist; then ran the appropriate non-normal checklist. We also requested to return for landing. The pilot not flying then contacted the Flight Attendant; Operations; and addressed the passengers about our condition/return. During this time; I monitored the battery temp; and noticed an increase in temperature 8 more degrees; to 72 degrees C. At this time I declared an emergency; and asked for the ARFF vehicles to follow us to the gate after landing. Once at the gate; we deplaned; and after contacting Maintenance; instructed the fire personnel to disconnect the aux battery. The Main and Aux batteries were removed and replaced by Maintenance uneventfully. There was no obvious reason why the battery overheated. All voltages/loads appeared normal during the event. OAT was 29 degrees C.