Japan transport ministry official Shigeru Takano said Monday the probe into battery-maker GS Yuasa was over for now as no evidence was found it was the source of the problems.
Ministry officials said they will inspect Kanto Aircraft Instrument Co. on Monday as part of the ongoing investigation. It makes a system that monitors voltage, charging and temperature of the lithium-ion batteries.
All 50 of the Boeing 787s in use around the world are grounded after one of the jets operated by All Nippon Airways made an emergency landing in Japan earlier this month when its main battery overheated. Earlier in January, a battery in a Japan Airlines 787 caught fire while parked at Boston's Logan International Airport.
GS Yuasa shares jumped on the news it is no longer being investigated, gaining nearly 5 percent in Tokyo trading. The issue had plunged 12 percent after the battery problems surfaced in Japan.
Ministry officials stopped short of saying that Kanto's monitoring system was under any special scrutiny, saying it was part of an ongoing investigation.
"We are looking into affiliated parts makers," Takano said. "We are looking into possibilities."
Kyoto-based GS Yuasa declined to comment, noting that the investigation was still underway.
Hideaki Kobayashi, spokesman for Kanto Aircraft, based in Fujisawa, southwest of Tokyo, declined comment. He said it was too early to tell whether its system was behind the problems.
Last week, U.S. federal investigators said the JAL battery that caught fire showed evidence of short-circuiting and a chemical reaction known as "thermal runaway," in which an increase in temperature causes progressively hotter temperatures. It's not clear to investigators which came first, the short-circuiting or the thermal runaway.
Deliveries of the jet dubbed the Dreamliner were three years behind schedule because of manufacturing delays. Much of the aircraft is made by outside manufacturers, many of them major Japanese companies who make about 35 percent of the plane.
It is the first jet to make wide use of lithium-ion batteries, the kind usually found in laptops and other gadgets. They are prone to overheating and require additional systems to avoid fires.
Investigators have been looking at the remnants of the ANA flight's charred battery, but it is unclear whether the battery or a related part was behind its overheating. Investigators have said the ANA battery and the JAL battery did not receive excess voltage.
Japanese carrier All Nippon Airways was the "launch customer" for the 787, and has been forced to cancel services - 643 domestic flights through Feb. 12, affecting 69,000 passengers, and 195 international flights through Feb. 18, affecting 13,620 passengers.
Japan Airlines, which has fewer 787s than ANA, has deployed other aircraft in its fleet, minimizing its flight cancellations.
Boeing, which competes against Airbus of France, has halted 787 deliveries. Boeing has orders for more than 800 of the Dreamliner planes.
The 787 is the first airliner made mostly from lightweight composite materials that boost fuel efficiency. It also relies on electronic systems rather than hydraulic or mechanical systems to a greater degree than any other airliner.
Analysts say customers won't come back to the 787 unless its safety is solidly assured.
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