Detailed examination of the auxiliary power unit (APU) battery of the plane revealed that two of its eight cells were misshapen. An initial inspection found the unit to be intact after the All Nippon Airways (ANA) plane made an emergency landing last month.
"Details such as how much they bulged remains unknown," said the official from the Japan Transport Safety Board (JTSB).
In their probe, Japanese officials have focused on the ANA Dreamliner's main lithium-ion battery, which was severely damaged by what they believe to have been a build up of heat that resulted in uncontrollably high temperatures.
Investigators have released a picture showing the blackened remains of the main battery in the ANA plane.
In contrast, the APU battery was initially thought to be intact.
The APU on a Japan Airlines flight that had landed in Boston in the United States was badly damaged in an onboard fire in January, marking the beginning of a torrid period for Boeing's flagship aircraft, which is still grounded worldwide.
ANA and JAL, major customers of the aircraft with more than 100 combined orders, have been hit hard by the grounding, slashing hundreds of flights, affecting tens of thousands of passengers.