What was found
AIRCRAFT WAS RECEIVED WITH EXTERNAL POWER CONNECTED, THE EXTERNAL POWER WAS CUT AND THE APU WAS STARTED, THE APU AUTOMATICALLY SHUT OFF, NOT BEING POSSIBLE TO ENERGIZE THE AIRCRAFT. MAINTENANCE PERFORMED AN INSPECTION, AND DETECTED THE MAIN BATTERY WITH LOW VOLTAGE, THE BATTERY WAS REPLACED IAW AMM DMC-B787-A-24-31-01-02D-520A-A AND AMM DMC-B787-A-24-31-01-02D-720A-A, TESTED CORRECT.