As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
RETURNED TO THE FIELD AT SFO AFTER THE APU WOULD NOT START AT TOP OF CLIMB. APU WAS REMOVED FROM AIRCRAFT AND TESTED IN APU TEST CELL. NO FAULT FOUND WITH APU S/N 61832. PASSED ALL TEST AND RETURNED TO SERVICE. AFTER APU REPLACEMENT, MECHANIC FOUND APU BATTERY LOW AND APU BATTERY CHARGER CIRCUIT BREAKER BROKEN. APU BATTERY WAS REPLACED AND OPERATION CHECKED OK.