Japanese investigators have found swelling in the lithium-ion cells of the auxiliary power unit battery on the All Nippon Airways Boeing 787-8 that was involved in an emergency landing on 16 January.
The Japan Transport Safety Board (JTSB) has confirmed that the team investigating the emergency landing at Takamatsu had found "slight swelling" on the lithium-ion battery cells, but there had been no internal damage to the batteries.
It adds that two out of eight cells in the battery unit showed some bumps, and that the JTSB would investigate further to determine if it was a regular occurrence.
ANA acknowledged the JTSB report but could not provide any further comment on the batteries or the investigation into the 787.
The APU batteries are located in an aft bay, away from the main battery bank where one of the batteries malfunctioned after experiencing a thermal runaway that forced the ANA 787 to make its emergency landing.
It is, however, the same battery bank where a similar incident of thermal runaway occurred on a Japan Airlines 787 that was parked at Boston's Logan airport on 7 January.
The two events led to the grounding of the global fleet of 51 787s from 16 January. Boeing is now working on an interim solution to the battery issue that could allow the grounding to be lifted, ahead of a permanent fix to the problem.
The US National Transportation Safety Bureau is also investigating potential short-circuiting issues on the 787 batteries, but said on 6 February that it is still some weeks away from determining the root cause of the incidents.