Boeing 787 Battery Troubles Still Unidentified

TOKYO--After a 20-month investigation, Japan's transportation safety authority said in a report Thursday it couldn't identify the precise cause of a battery malfunction that prompted an emergency grounding of a Boeing Co. 787 Dreamliner in Japan last year.

In a report, the authority said that instead of one single major defect, a number of factors apparently contributed to a short-circuiting that burned the main battery on an All Nippon Airways-operated plane in 2013.

Investigators say that the severe damage to the battery made it a difficult to analyze the accident.

The Dreamliner is the first commercial airliner to use lithium ion batteries as its main power supply. The batteries were supplied by Japan's GS Yuasa Corp.

The report requested that Boeing continue its probe into the accident, and that the U.S. Federal Aviation Administration step up safety checks on such batteries in the future.

The January 2013 incident occurred when the Dreamliner experienced a malfunction of its main battery and was forced to make an emergency landing. The battery was later found to be severely damaged from overheating.

The trouble came just days after a Dreamliner operated by Japan Airlines reported smoke from its battery while it was parked in Boston. The incidents prompted a grounding of the global 787 fleet for three and a half months.

Almost a year later, another battery overheating occurred on an ANA plane parked in a Tokyo airport.

The fact that all three cases took place in January led Japanese investigators to believe that cold temperatures likely played a part in the malfunctioning.

The report speculated that a phenomenon called "deposition" was behind the short-circuits. This happens when lithium metal builds up in cold temperatures and forms needlelike objects in the battery solution.

Deposition alone, however, is unlikely to cause such major short-circuits, the report also said. It suspect other factors also played a part, such as voltage spikes during a discharging of the battery.

The incident was made worse after a failure in one cell cascaded across the others inside the battery, the report said. As the cell overheated, it became inflamed and came into contact with other cells, creating even bigger short-circuits.

The possibility this happening wasn't properly recognized by either the FAA or by the manufacturer, the report said.

Boeing Japan wasn't immediately available for comment.

Write to Mitsuru Obe at [e-mail]

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