Cells in a second lithium-ion battery on a Boeing Co 787 Dreamliner forced to make an emergency landing in Japan last month showed slight swelling, a Japan Transport Safety Board (JTSB) official said on Tuesday.
The jet, flown by All Nippon Airways Co <9202.T>, was forced to make the landing after its main battery failed.
"I do not know the exact discussion taken by the research group on the ground, but I heard that it is a slight swelling (in the auxiliary power unit battery cells). I have so far not heard that there was internal damage," Masahiro Kudo, a senior accident investigator at the JTSB said in a briefing in Tokyo.
Kudo said that two out of eight cells in the second battery unit showed some bumps and the JTSB would continue to investigate to determine whether this was irregular or not.
The plane's auxiliary power unit (APU) powers the aircraft's systems when it is on the ground. National Transportation Safety Board investigators in the United States are probing the APU from a Japan Airlines <9201.T> plane that caught fire at Boston's Logan airport when the plane was parked.
The U.S. Federal Aviation Authority grounded all 50 Boeing Dreamliners in commercial service on January 16 after the incidents with the two Japanese owned 787 jets.
The groundings have cost airlines tens of millions of dollars, with no solution yet in sight.
Boeing rival Airbus said last week it had abandoned plans to use lithium-ion batteries in its next passenger jet, the A350, in favor of traditional nickel-cadmium batteries.
Lighter and more powerful than conventional batteries, lithium-ion power packs have been in consumer products such as phones and laptops for years but are relatively new in industrial applications, including back-up batteries for electrical systems in jets.
(Reporting by Mari Saito; Editing by Richard Pullin)