What was found
DURING TROUBLESHOOTING FOR THE APU BATTERY NOT CHARGING, THE CIRCUIT BREAKER, C302, WAS FOUND TRIPPED OPEN AND THREE WIRES W238-001-18, W238-002-18, W238-003-18 OVER THE CENTER TANK WERE FOUND CHAFED AND SHORTED TOGETHER. THE WIRES WERE REPAIRED AND THE BATTERY CHARGER OPERATIONALLY CHECKED GOOD. THE CAUSE OF THE WIRE CHAFING WAS A FOREIGN OBJECT LAYING ON THE WIRE. THE AIRPLANE WAS REMOVED FROM SERVICE (GROUNDED) DURING THE TROUBLESHOOTING AND REPAIR.