A B767-300 IN CRUISE AT FL310 DECLARED AN EMER AND DIVERTED DUE TO LOSS OF ALL ELECTRICAL POWER. MAIN SHIP'S BATTERY ONLY POWER AVAILABLE.

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LOST ALL ACFT POWER DURING CRUISE. ACFT PANEL APPEARED NORMAL. BATTERY DISCHARGE LIGHT CAME ON. HMG DID NOT ACTIVATE AS IT SHOULD HAVE. DECLARED EMER AND LANDED AT ZZZZ. LANDING AND ROLL OUT BOTH NORMAL. FSI CONDUCTED 4-WED-04 VIA CONFERENCE CALL WHILE IN ZZZZ. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS OPERATED FOR 41 MINS ON THE BATTERY UNTIL THE AIRPLANE WAS ON THE GND AND ONLY A FEW LIGHTS WERE OPERATIVE; BATTERY DISCHARGE LIGHT; ANTI-SKID WARNING AND EICAS. THE RPTR SAID THE PROB WAS TRACED TO A GROUNDING BRACKET LOCATED IN THE ELECTRICAL AND ELECTRONICS COMPARTMENT AND WAS THE COMMON GND FOR THE AIRPLANE. THE RPTR SAID THE BRACKET WAS FOUND RIVETED TO STRUCTURE WITH BOTH SURFACES PAINTED. THE RPTR STATED THE AIRPLANE HAD A PREVIOUS MAINT HISTORY OF UTILITY BUS FAILURES. THE RPTR STATED THE BATTERY WAS POWERING THE AIRPLANE FOR 41 MINS; 11 MORE MINS THAN THE MANUAL STATES. THE RPTR SAID IF THIS INCIDENT OCCURRED OVER WATER; IT WOULD HAVE BEEN NECESSARY TO DITCH. THE RPTR STATED THE HYDRAULIC MOTOR GENERATOR DID NOT OPERATE WITH THE LOSS OF L AND R AC BUSSES. THE RPTR SAID THE CARRIER HAS CHECKED ALL THE FLEET GROUNDING BRACKETS FOR PAINT ON PAINT SURFACES AND SECURITY.