What was found
AIRCRAFT WAS CONNECTED TO EXTERNAL AIRCRAFT POWER CART, AND POWERED UP. A LOUD POP WAS HEARD COMING FROM AFT EQUIPMENT BAY OF AIRCRAFT. DURING INVESTIGATION, ANOTHER POP WAS HEARD. ACCESSED APU BATTERY. FOUND IT TO BE DAMAGED AND SMOKING. FOUND SEVERAL CELLS BURNED AND HEAT BLANKET BURNED, ALSO. SUSPECT BATTERY CHARGER TO BE FAULTY. BOTH COMPONENTS REPLACED. REMOVED ITEMS, SENT FOR EVALUATION. NO DAMAGE TO AIRCRAFT. THE COCKPIT INDICATION AT TIME OF INCIDENT SHOWED (APU BATT FAIL) TEMPERATURE AND VOLTAGE WERE NORMAL.