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, the new Boeing jetliner’s biggest customer to date, and Japan Airlines Co Ltd 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. 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.