Updated at 4:26 p.m. ET
TOKYO 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.
Substantial progress has been made in developing ways to mitigate the risk of a battery meltdown and fire, sources told CBS News Wednesday. Key Boeing and FAA officials are expected to meet Friday at FAA headquarters in Washington, the sources said.
The FAA could certify a fix to the battery system and end its grounding order before the National Transportation Safety Board and its Japanese counterpart complete their respective investigations of the two incidents, the sources said.
"Decisions to return the airplane to flight will be made by the FAA and only after Boeing has demonstrated to them that the proposed solution is adequate," NTSB spokeswoman Kelly Nantel told CBS News. "The NTSB continues to investigate the cause of the short circuiting in the JAL (Japan Airlines) battery."
As CBS News investigative correspondent Sharyl Attkisson reported earlier this month, federal investigators revealed the fire aboard the 787 in Boston began with a short circuit in one of the lithium ion battery's eight cells.
That caused a dangerous condition known as "thermal runaway," where a short spreads to other cells in an uncontrolled chemical chain reaction, Attkisson reports. The fire topped 500 degrees Fahrenheit.
It's something Boeing's development studies didn't predict.
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.