The U.S. National Transportation Safety Board ruled out on Sunday excess voltage as the cause of a battery fire on the Boeing Co 787 Dreamliner jet operated by Japan Airlines Co (JAL) <9201.T> at Boston airport this month.
Last week, governments across the world grounded the Dreamliner while Boeing halted deliveries after a problem with a lithium-ion battery on a second 787 plane, flown by All Nippon Airways Co (ANA) <9202.T>, forced the aircraft to make an emergency landing in western Japan.
"Examination of the flight recorder data from the JAL B-787 airplane indicates that the APU (auxiliary power unit) battery did not exceed its designed voltage of 32 volts," the NTSB said in a statement forwarded by a Boeing Japan representative.
On Friday, a Japanese safety official told reporters that excessive electricity might have overheated the battery in the ANA-owned Dreamliner which was forced to make the emergency landing at Japan's Takamatsu airport last week.
U.S. investigators have examined the lithium-ion battery that powered the APU, where the battery fire started in the JAL plane, as well as several other components removed from the airplane, including wire bundles and battery management circuit boards, the NTSB statement said.
Japan Transport Safety Board said it was aware of the NTSB report and would take the U.S. statement into consideration in its probe.
"There's nothing more I can add at this point as we still haven't started our investigation into the battery here," JTSB inspector Hideyo Kosugi told Reuters.
"The NTSB's investigation started earlier. We still haven't taken X-rays or CT-scans of the battery. In our case, both the battery and the surrounding systems are still stored in (Tokyo's) Haneda (Airport) as the third party organisation where the Japanese investigation would take place still has not been chosen."
On Tuesday, the U.S. investigating group will convene in Arizona to test and examine the battery charger and download non-volatile memory from the APU controller, the NTSB added.
Japan's GS Yuasa Corp <6674.T> makes batteries for the Dreamliner, while France's Thales produces the control systems for the battery.
Boeing said on Friday it would continue building the carbon-composite 787, but deliveries were on hold until the U.S. Federal Aviation Administration approved and implemented a plan to ensure the safety of potentially flammable lithium-ion batteries.
In Washington, the top U.S. transportation official Ray LaHood said the 787, which has a list price of $207 million, would not fly until regulators were "1,000 percent sure" it was safe.
Japan is the biggest market so far for the 787, with ANA and JAL operating 24 of the 290-seat wide-bodied planes. Boeing has orders for almost 850 of the planes.
(Additional reporting by Kentaro Sugiyama, Yoshiyuki Osada and Yuka Obayashi; Editing by Daniel Magnowski)
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