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.
LOT Polish Airlines is losing $50,000 a day due to the grounding of its two 787s, according to government information made public on Wednesday.
LOT, a deeply indebted state-run airline, was the first European one to get the Dreamliners. Deputy Treasury Minister Rafal Baniak put total losses due to the grounded planes Wednesday at $2.5 million. The airline also is awaiting six more 787s to be delivered.
Boeing has not said how much the 787 grounding will cost it.
Imperial Capital analyst Ken Herbert estimated last week that it could cost Boeing $25 million per month in direct costs, with the total price tag climbing past $1 billion, including spending to fix the problem and expenses for delayed deliveries.