Japan said on Friday the battery in a Dreamliner jet that spewed white smoke last month could have reached temperatures as high as 660 Celsius (1,220 Fahrenheit).
The figure came in an report released by the transport ministry after Japan Airlines grounded one of its Dreamliner planes when the smoke was seen outside the cockpit window during maintenance at Tokyo's Narita airport.
It was the latest in a series of problems that have beset the aircraft, including a months-long global grounding over battery problems last year.
According to the report, one of the eight lithium-ion cells in the plane's battery system "swelled and electrolytic solution was sprayed" out of the battery, a ministry official said.
An image released by the ministry showed some blackened electrolyte solution coming from one of the safety valves.
"We estimate that temperatures inside the cell possibly reached 660 Celsius as the aluminium electrode melted," the official said.
It is still unknown what actually caused the overheating, the official said, adding that the ministry would continue investigating the case with the help of Boeing.
The US aviation giant admitted last April that despite months of testing it did not know the root cause of the battery problems, but rolled out modifications it said would ensure the issue did not recur.
The measures comprised redesigning the battery and charger system and adding a steel box to prevent burning.
Since then, Dreamliners have experienced a series of minor glitches, including a fault with an air pressure sensor and the brake system.