[TOKYO] Japan Airlines Co (JAL) is investigating what caused a battery on a Boeing Co 787 Dreamliner to smoke during pre-flight maintenance, a year after a fire on one of the carrier's jets helped spur the grounding of the global fleet.

Mechanics found that one of eight battery cells had vented liquid through a safety valve on Jan 13, while the others were intact, said Seiji Takaramoto, a JAL spokesman. Instruments in the cockpit showed a possible fault in a main battery and a main battery charger, he said.

The incident is the first sign of a battery failure since Boeing redesigned the units after a pair of meltdowns on 787s, one flown by JAL and the other by ANA Holdings Inc.