The Transport Safety Board said in a report that the battery of the aircraft's auxiliary power unit was incorrectly connected to the main battery that overheated, although a protective valve would have prevented power from the APU from doing damage.
Flickering of the plane's tail and wing lights after it landed and the fact the main battery was switched off led the investigators to conclude there was an abnormal current traveling from the APU due to miswiring.
The agency said that more analysis was needed to determine what caused the main battery to overheat and emit the smoke that prompted the Jan. 16 emergency landing of the ANA domestic flight and the worldwide grounding of Boeing (BA) 787 jets. They said they are consulting Boeing about the issue.
The 787, dubbed the Dreamliner by Boeing, is the first airliner to make extensive use of lithium ion batteries, which are lighter weight, charge faster and contain more energy than conventional batteries similar in size. However, the batteries also are more prone to overheating and catching fire.