(Reuters) — Boeing Co. gave U.S. aviation regulators its plan to fix the volatile battery aboard its new 787 Dreamliner on Friday, even before investigators know what caused the batteries to overheat on two planes last month.

Boeing will not propose abandoning the lithium-ion batteries, however, and is not working on a backup or longer-term fix for the problem that has grounded its entire fleet of 50 787 Dreamliners, three sources familiar with the plan said.

The proposal, made to the U.S. Federal Aviation Administration, includes specific measures aimed at addressing possible causes of short circuits that prompted one of the lithium-ion batteries on board a 787 to catch fire at a Boston airport in January, the sources said. A second battery smoldered during a flight in Japan a few days later, leading to an emergency landing and evacuation.

The proposal includes insulation between the cells of the battery and a stronger, stainless steel box with a venting tube to contain a fire and expel fumes outside the aircraft should a battery catch fire again, the sources said.

"I have talked to a number of people who are working directly on these batteries. No one is on the Plan-B team," said a person familiar with Boeing's efforts who was not authorized to speak publicly about them.

Boeing's proposal to the FAA is not a temporary "band-aid" that would be supplanted by another solution later, said a second source, who also was not authorized to speak publicly.

The FAA did not respond to requests for comment.

%%BREAK%%

Boeing declined to comment specifically on its proposal, and reiterated that hundreds of engineers and technical experts are working "around the clock" to return the 787 fleet to service. "Everyone is working to get to the answer as quickly as possible and good progress is being made," spokesman Marc Birtel said.

Five weeks after U.S. authorities grounded the worldwide fleet of 787s, U.S., Japanese and French investigators are still not certain what caused the battery fire aboard an All Nippon Airways 787 in Boston and an overheated, smoking battery on a Japan Airlines 787 in Japan.

The National Transportation Safety Board is still investigating the Boston fire and the Japan Transportation Safety Board is investigating the battery failure in Japan. Neither have found a root cause for the problems.

The sources said it is possible the NTSB might never determine the root cause.