The Japan Transport Safety Board told Reuters that the second lithium-ion battery on a Boeing 787 that made an emergency landing in January showed swelling.
The All Nippon Airways flight made an emergency landing on January 16th after its main battery failed.
JTSB senior accident investigator Masahiro Kudo said the research group found slight swelling in the auxiliary battery, but has not yet found any internal damage.
According to Reuters, Kudo explained that two of the eight cells showed some bumps. The investigation is underway to determine if that was irregular or not.
Currently, 50 Dreamliners operated by seven airlines in six countries have been grounded while the issues with the battery are resolved.