The Transport Safety Board said in a report that the battery for 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 auxiliary unit from causing 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 auxiliary power unit due to miswiring.
The agency said 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 787 jets. They said they are consulting Boeing about the issue.
The Federal Aviation Administration and aviation authorities in other countries grounded 787 fleets because of the ANA incident, which followed a battery fire earlier in January in a 787 parked in Boston.
The 787, dubbed the Dreamliner by Boeing, is the first airliner to make extensive use of lithium ion batteries, which are lighter in 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.
Also Wednesday, a congressional official says Boeing will propose to federal regulators a plan to temporarily fix problems with the 787 Dreamliner's batteries.
The official said Boeing Commercial Airplanes CEO Ray Conner is expected to present the plan to Michael Huerta, head of the Federal Aviation Administration, in a meeting later this week. The official spoke on condition of anonymity because he wasn't authorized to speak publicly about the plan.