As the mystery deepened surrounding the new 787 Dreamliner battery system, U.S. federal investigators were set to conduct more tests aimed at eventually returning Boeing's grounded aircraft to the sky.
Tuesday's NTSB tests will be conducted at a special facility Arizona.
Tests conducted in Washington show that a lithium-ion battery that caught fire aboard an empty JAL Boeing 787 Dreamliner in Boston this month was not overcharged, the National Transportation Safety Board said Sunday.
Suspicions about the batteries and a possible fire risk prompted the U.S. Federal Aviation Administration and other nations to ground all 50 Dreamliners worldwide last week, pending an investigation of the Boston fire and other reports of mechanical problems.
The aircraft, which costs about $200 million and debuted 15 months ago, is the first major airliner to be rolled out in years. Among U.S. carriers, only United Airlines currently operates the Dreamliners. It has six. The airline began the nation's first domestic 787 routes in November.
Sunday's NTSB announcement "doesn't bode as well for a quick fix as Boeing would have liked," said John Goglia, a former member of the NTSB, the nation's top aviation investigation agency. "It's one step in the process. It's not great news, but it's not bad news either."
The probe is focusing on the 787's cutting-edge lithium ion battery system, which is the most extensive airliner battery system of its kind. Other airliner battery systems are mostly powered by more traditional and heavier nickel cadmium cells.
On Tuesday in Arizona, the NTSB team plans to test the battery charger and download computer memory components from the auxiliary power unit, or APU, controller, the agency said. The APU powers non-propulsion systems aboard an airliner.
The examination of the Boston plane's flight recorder data indicated the APU battery did not exceed its designed voltage of 32 volts, according to Sunday's NTSB statement. It didn't appear to be overcharged, which might overheat the unit and cause a fire. But experts pointed out to CNN that there was no mention in the statement about how quickly the JAL 787 battery was discharging.
Discharging the battery too quickly, or with too low voltage, can also cause it to overheat, said University of Dayton professor Raul Ordonez, an aircraft electrical and computer engineer who spent time observing Dreamliner development at Boeing's Seattle headquarters.
Investigators in Washington have taken X-rays and CT scans of the lithium-ion battery that caught fire in Boston, the NTSB said, and they have taken the battery apart and examined some of its individual cells.
The agency said it has also examined several other components from the plane, including wire bundles and battery management circuit boards.
"The fact that the NTSB is basically looking at every component around the battery, including the computer hardware and the (memory) software, means that they have no idea yet about a culprit and (they) suspect everything," Ordonez said.
The batteries in question are manufactured by Japan's GS Yuasa, under a subcontract to France-based Thales, Boeing said. Kyoto-based GS Yuasa says it has dispatched a team to Washington to help in the investigation.
Boeing is using the lithium-ion batteries to electronically assist some of the functions that were previously performed using hydraulics. A lighter plane is more fuel efficient, which is one of the 787's main selling points.
There is no need to drain lithium-ion batteries fully before recharging, meaning less maintenance, though they can catch fire if overcharged.
The grounding resulted from recent mechanical and other glitches culminating with the fire in Boston and an emergency landing in Japan last week prompted by a battery alarm. Some of the 129 people aboard reported a burning smell in the cabin, and an alarm indicated smoke in an electrical compartment.
The FAA issued a directive last week ordering all U.S. 787s not to fly until the problems are resolved. Other nations quickly followed suit, including the Polish carrier LOT, which told CNN in a statement it is considering seeking compensation from Boeing for costs associated with the grounding.
"We are preparing for such a solution. At this moment it's too early to reveal any news." the statement said.
Airbus uses lithium-ion batteries to power some systems aboard its A350 airliners. A spokeswoman said in a statement to CNN that Airbus "will carefully study any recommendations that come out of the 787 investigation and evaluate whether they apply to the A350 XWB."
Boeing said Friday it will not deliver any Dreamliners to its customers as it works with the FAA over the battery concerns.
"We need to get the bottom of this," said Goglia. "We need to get comfortable with flying these airplanes again."