As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING THE APU START, THE APU BATT VOLTAGE FALLS FROM 30V TO 3V WITH AN APU BATTERY FAULT LIGHT ILLUMINATED. THE INVESTIGATION CLARIFIED THAT THE APU BATTERY GROUNDING STUD ON THE AFT FUSELAGE FRAME STA 925.00 WAS BURNED AND MELTED.