Japan’s transportation authority failed to find the cause of a lithium-ion battery meltdown on a Boeing Co. 787 almost a year after the incidents led to a three-month grounding of the fleet by U.S. and Japanese regulators.

A build-up of lithium, a metal fragment or damage to a cell separator may have caused a short circuit in the battery, the Japan Transport Safety Board said in a statement today. The board’s probe focused on a battery that overheated aboard an ANA Holdings Inc. flight last year.

Boeing redesigned the battery made by Kyoto-based GS Yuasa Corp. before restarting 787 flights, with no battery-related groundings by regulators since then. Two meltdowns aboard aircraft, including one on a Japan Airlines Co.-operated 787 on the ground in Boston, added to overheating incidents reported for lithium-ion batteries used in electric cars, notebook computers and mobile phones.

“Looking at how flights have been going, I’d say the problem has been sorted,” Norihiro Goto, Japan Transport Safety Board chief, told reporters. “We may not have found the cause, but there is already a working solution.”

The groundings cost ANA 8 billion yen ($73 million) in lost sales in the first fiscal quarter last year, the airline said, and were the longest for a large commercial aircraft by U.S. and Japanese regulators since the 1950s. JAL said its sales were cut by 4.8 billion yen in the period because of the incident.

Cold Air

ANA was little changed, gaining 0.1 percent as of 9:23 a.m. in Tokyo trading today. Japan Airlines fell 0.5 percent, while the Nikkei 225 Stock Average rose 0.9 percent.

Cold air, which can increase lithium build-up inside a battery, may have helped cause the overheating, since the accidents happened in January, the JTSB report said.

Boeing redesigned the battery to include more protection around individual cells to contain any overheating and added a steel case and a tube that would vent any fumes to outside the aircraft’s fuselage.

The U.S. National Transportation Safety Board hasn’t concluded what caused a battery aboard a Japan Airlines 787 on the ground in Boston to fail on Jan. 7, 2013.

A Japan Airlines 787 emitted smoke during preflight maintenance in January this year and mechanics found that one of the eight battery cells had vented liquid through a safety valve.

The Dreamliner’s success is crucial for the two Japanese carriers as they have ordered 125 of the plastic-composite jets with ANA being the world’s largest airline customer for the plane. Tokyo-based ANA had 32 Dreamliners in its fleet at the end of last month, while JAL had 15, according to Boeing.

Earlier this year U.K. investigators linked the fire on board a parked Boeing Co. 787 Dreamliner at London’s Heathrow Airport last year to faulty wiring in the plane’s emergency locator transmitter.

To contact the reporters on this story: Chris Cooper in Tokyo at ccooper1@bloomberg.net; Kiyotaka Matsuda in Tokyo at kmatsuda@bloomberg.net. To contact the editors responsible for this story: Dave McCombs at dmccombs@bloomberg.net; Frank Longid at flongid@bloomberg.nett. 

Photo Credit: All Nippon Airway's Boeing 787 planes sit on a tarmac at Haneda International Airport in Tokyo Friday, April 26, 2013. Kyodo News / Associated Press