Even before two battery failures led to the grounding of all Boeing 787 Dreamliner jets, the planes’ lithium-ion batteries had experienced repeated problems that raised questions about their reliability, The New York Times reports.

Executives at All Nippon Airways, the jets’ biggest operator, told the Times in an interview Tuesday that it had replaced 10 of the batteries in the months before the two incidents that led regulators around the world to ground the jets.

The airline said it had told Boeing (NYSE:BA) of the replacements as they occurred but had not been required to report them to regulators because no flights were canceled or delayed. National Transportation Safety Board officials said Tuesday that the replacements were now part of their inquiry.

The airline also, for the first time, explained the extent of the previous problems, which underscore the volatile nature of the batteries and cast more doubts on whether aircraft will be able to use them safely.

In five of the 10 replacements, All Nippon said the main battery had showed an unexpectedly low charge. An unexpected drop in a 787’s main battery also occurred on the All Nippon flight that had to make an emergency landing in Japan on Jan. 16.

The airline also revealed to the Times that in three instances, the main battery failed to operate normally and had to be replaced along with the charger. In other cases, one battery showed an error reading and another, used to start the auxiliary power unit, failed. All the events occurred from May to December of last year. And all the batteries were returned to their maker, GS Yuasa.

Kelly Nantel, an NTSB spokeswoman, said investigators had only recently heard that there had been “numerous issues with the use of these batteries” on 787s. She said the board had asked Boeing, All Nippon and other airlines for information about the problems.

Boeing, based in Chicago, has said repeatedly that any problems with the batteries can be contained without threatening the planes and their passengers.

But in response to All Nippon’s disclosures, Boeing officials said the airline’s replacement of the batteries also suggested that safeguards to prevent dangerous overheating of the batteries might have kicked in.

Boeing officials also acknowledged that the new batteries were not lasting as long as intended. But All Nippon said that the batteries it replaced had not expired.

Meanwhile, the NTSB said Tuesday it was carrying out a detailed, microscopic investigation of a battery that caught fire on a 787 Dreamliner in Boston, Reuters reported.

All 50 Boeing Dreamliners remain grounded around the world, as the U.S., Japanese and French governments continue to investigate that fire and a separate battery-related incident that forced another 787 to make an emergency landing in Japan.

The NTSB said experts at the U.S. Naval Surface Warfare Center laboratories were looking at a second, undamaged lithium-ion battery pulled from the same Japan Airlines (Tokyo: 9201) plane that caught fire in Boston for signs of in-service damage and manufacturing defects. Both batteries were built by GS Yuasa (Tokyo: 6674), a Japanese company.

Boeing's shares closed 0.5 percent lower at $73.65 on the New York Stock Exchange on Tuesday. Investors are looking for news about how long the probe will take when Boeing reports its fourth quarter earnings on Wednesday.

A one-month delay in 787 deliveries could cost Boeing $1.2 billion in revenue this year, Zafar Khan, an analyst at Society Generale, told Reuters. He has a "sell" rating on the stock.