A B777 OVERWATER FLT EXPERIENCES AN OVERHEATING MAIN BATTERY WHICH CAN ONLY BE DISCONNECTED AS AN EMER PROC WITH THE FLT MAKING AN EMER DIVERSION AND LNDG AT CYYT; NF.

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I WAS FLYING AS PURSER/LEAD OF FLT ATTENDANT CREW. CAPT CALLED TO HAVE ME IN COCKPIT FOR 'BATHROOM BREAK' AND TOLD ME OF A SIT WITH THE MAIN BATTERY OVERHEATING. FLC HAD BEEN IN TOUCH WITH ZZZ MAINT TO NO AVAIL -- NO PROC FOR MAIN BATTERY OVERHEATING. CAPT VISUALLY CHKED BATTERY ON CARGO LEVEL AND FOUND IT SMOKING; SO HE DISCONNECTED THE WIRES; AND TEMPORARILY RECTIFIED PROB. BACK IN THE COCKPIT AS WE SPOKE; THE HEAT INDICATOR ROSE AGAIN AND A SYS WARNING LIGHT CAME ON THE PANEL; FORCING HIM TO DECLARE A MAYDAY. WE HAD 20 MINS TO PREPARE CABIN FOR EMER LNDG -- ACCOMPLISHED CHKLIST IN 10 MINS. JUST PRIOR TO LOWERING GEAR; CAPT CALLED TO INFORM WE WOULD NOT NEED TO EVAC. LANDED WITHOUT INCIDENT (AND NO MORE SMOKE IN CABIN). YYT FIRE CREW DECLARED US SAFE TO TAXI. AS FAR AS HUMAN FACTORS ARE CONCERNED; WHILE I GATHERED CREW TO INFORM THEM OF THE EMER; ETC; A FEW OF OUR COACH PAX COULD SEE BY SIGHT AND AIRSHOW THAT WE WERE NO LONGER CONTINUING TO NEW YORK AND STARTED A WAVE OF PANIC AND GRABBING LIFE VESTS. LUCKILY; WE CONTAINED IT TO THAT ZONE; BUT I DID ELECT TO MAKE A SECOND PA TO MAKE SURE THEY KNEW HOW TO OPERATE THE VEST AND; MORE IMPORTANTLY; WHEN NOT TO. (WE HAD SHUT DOWN VIDEO SYS WHEN THE CAPT INITIALLY EXPLAINED OUR SIT IN HIS ANNOUNCEMENT; BUT IT WAS TOO LATE.) APPROX 40 VESTS WERE OUT WHICH THE PAX WOULD NOT TAKE OFF UNTIL WE WERE SAFELY ON THE GND.