Japan's two biggest airlines replaced lithium-ion batteries on their Boeing Co 787 Dreamliners in the months before separate incidents led to the technologically advanced aircraft being grounded worldwide due to battery problems.
Comments from both All Nippon Airways <9202.T>, the new Boeing jetliner's biggest customer to date, and Japan Airlines Co Ltd <9201.T> point to reliability issues with the batteries long before a battery caught fire on a JAL 787 at Boston's airport and a second battery was badly charred and melted on an ANA domestic flight that was forced into an emergency landing.
ANA said it changed 10 batteries on its 787s last year, but did not inform accident investigators in the United States because the incidents, including five batteries that had unusually low charges, did not compromise the plane's safety, spokesman Ryosei Nomura said on Wednesday.
JAL also replaced batteries on the 787 "on a few occasions", said spokeswoman Sze Hunn Yap, declining to be more specific on when units were replaced or whether these were reported to authorities.
ANA did, however, inform Boeing of the faults that began in May, and returned the batteries to their manufacturer, GS Yuasa Corp <6674.T>. A spokesman for the battery maker declined to comment on Wednesday. Shares of the company fell 1.2 percent.
Boeing, in a statement, said battery replacements are not unusual for airplanes.
"We have not seen 787 battery replacements occurring as a result of safety concerns," the company said.
An NTSB spokesman said the board was aware of the reports of the prior battery problems and would review the data to see if it was relevant to the broader 787 probe.
Under aviation inspection rules, airlines are required to perform detailed battery inspections once every two years.
Officials are carrying out detailed tests on the batteries, chargers and monitoring units in Japan and the United States, but have so far made little headway in finding out what caused the battery failures.
Japan's transport ministry said the manufacturing process at the company which makes the 787 battery's monitoring unit did not appear to be linked to the problem on the ANA Dreamliner that made the emergency landing.
The NTSB said on Tuesday it was carrying out a microscopic investigation of the JAL 787 battery. Neither it nor the Japan Transport Safety Board has been able to say when they are likely to complete their work.
The global fleet of 50 Dreamliners - 17 of which are operated by ANA - remain grounded, increasing the likely financial impact to Boeing, which is still producing the aircraft but has stopped delivering them, and the airlines that fly the Dreamliner.
Boeing said on Wednesday that its 2013 financial forecast assumes no significant impact from the grounding. Boeing shares rose slightly in early trading and are down just 0.5 percent since the 787 was grounded.
ANA posts its earnings on Thursday. ANA shares rose 0.56 percent on Wednesday.
(Reporting by Tim Kelly, Dominic Lau, James Topham, Alwyn Scott and Andrea Shalal-Esa; Editing by Ian Geoghegan)