As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE AIRCRAFT WAS GROUNDED. DURING CLIMB WITH ELECTRICAL BACKUP SYSTEM ON, AIRCRAFT LOST BOTH BACK UP GENERATORS AND RECEIVED EICAS STATUS MESSAGE "ELEC BACKUP SYS". THE FLIGHT DECLARED AN EMERGENCY AND THE AIRCRAFT RETURNED TO KLAX. MAINTENANCE REPLACED THE BACKUP POWER SYSTEM FREQUENCY CONVERTER ASSEMBLY IAW AMM 24-25-09. CONTROL TEST AND POWER TEST IS NORMAL IAW AMM 24-25-09, NO FAULTS FOUND.