Japan's Transport Safety Board announced on February 20 that the All Nippon Airway's Boeing 787 had a miswiring. The main battery and the auxiliary battery were connected but they shouldn't have been connected. Boeing's initial blueprint for 787 had these batteries connected. The blueprint was later revised so that the batteries were separate, but that revision wasn't reflected in the ANA plane.
When I first read the AP article (2/20/2013) reporting on the finding, I thought these batteries were supposed to be connected, but somehow connected "improperly"; if only they were connected "properly" there would have been no problem:
TOKYO (AP) — A probe into the overheating of a lithium ion battery in an All Nippon Airways Boeing 787 that made an emergency landing found it was improperly wired, Japan's Transport Ministry said Wednesday.
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.
Then I found Asahi Shinbun article (2/20/2013) that corrected my misunderstanding:
The Transport Safety Board announced on February 20 that a design error in wiring was found in the ANA that made an emergency landing in Takamatsu Airport. The Board denied the error was related to the charred battery, but ANA's other two planes may have similar wiring errors.
In a Boeing 787 plane, the main battery is located in the front of the plane and the battery for the auxiliary power unit (APU) is in the back of the plane. In the ANA plane, the main battery was carbonized.
According to the Transport Safety Board, these two batteries are normally designed to be on separate circuits.
However, when the Board investigated the ANA plane, these two batteries were connected, and they were connected in Boeing's blueprint.
[According to the Transport Safety Board,] unintended current may run on the circuit if these two batteries are connected.
Three ANA planes including the one with the battery problem were some of the first 787 planes built by Boeing. Boeing revised the blueprint later to separate circuits for the two batteries. The Transport Safety Board is investigating why the revision was not reflected in the ANA plane.
Would the US FAA want to hear from ANA more on this? In addition to the blueprint revision not reflected, ANA changed the lithium-ion battery 10 times last year without reporting the incidents to the FAA.