A lithium-ion battery on a Boeing 787 that overheated during a flight was not overcharged, Japan's transport safety agency said.
Japan Transport Safety Board chairman Norihiro Goto said the All Nippon Airways jet's data recorder showed the main battery, used to power many electrical systems on the plane, did not exceed its maximum voltage in the incident earlier this month.
That contradicts an earlier assertion by the agency as it investigates with the US Federal Aviation Administration.
All 50 of the 787 Dreamliners Boeing has delivered to airlines were grounded after the emergency landing by the ANA flight in western Japan on January 16.
Boeing has also halted deliveries of new planes until it can address the electrical problems.
Mr Goto said the maximum voltage recorded for the battery was 31 volts, which was below its 32-volt limit.
But the data also showed a sudden, unexplained drop in the battery's voltage, he said.
Aircraft do not usually use the kind of lithium ion battery chosen for the 787, and investigators are still struggling to figure out what went wrong.
"It's not that it is difficult, but that we are not so familiar with it," Mr Goto said.
The Transport Safety Board said it will also study the aircraft's auxiliary battery and compare data from each.
Investigators from both sides are probing GS Yuasa, the maker of the charred battery, and are examining the battery using CAT scans at a facility of Japan's aerospace agency.
US investigators also said they found no evidence of overcharging in a battery that ignited on a Japan Airlines Boeing 787 as it sat on the tarmac at Boston's airport earlier this month.
A key U.S. Senate committee will hold a hearing in coming weeks to examine U.S. aviation safety oversight and the Federal Aviation Administration's decision to allow Boeing to use the highly flammable lithium-ion batteries on board the new Dreamliner, a congressional aide has said.
U.S., Japanese and French authorities are investigating the Boston and Japan casees in which lithium-ion batteries on board the new airliner failed.
The Dreamliner, with a list price of $207 million, is the world's newest airliner, a lightweight, advanced carbon-composite design that has more electrical power than any other aircraft and uses 20 per cent less fuel.
"Certainly the issues of FAA certification will be a key component of the aviation safety oversight hearing we're planning," an aide to the Senate Commerce, Science and Transportation Committee said.
The aide, who was not authorised to speak publicly, said committee chairman Senator John Rockefeller was "following the situation surrounding the Dreamliner and FAA's task force closely and he thinks the FAA and (Department of Transportation)are examining the issue carefully."
The House Transportation and Infrastructure Committee is also keeping a close eye on the 787 investigations and the issue of FAA oversight, congressional aides said, although no formal hearings were planned at this point.
Boeing officials have briefed both oversight committees and other key lawmakers about the matter, a Boeing spokesman said.
The Senate committee had already been planning to conduct "substantial and aggressive oversight" of aviation safety during the first quarter, but would now look closely at the 787 incidents and FAA oversight as part of that process, the committee aide said.
Problems with the 787's lithium-ion battery have sparked questions about why the FAA in 2007 granted Boeing a "special condition" to allow use of the batteries on the plane, despite the fact that they are highly flammable and hard to extinguish if they catch fire.
Boeing designed a special system that was supposed to contain any such fire and vent toxic gasses outside the plane, but the two recent incidents have raised questions about whether that was a good decision.
It remains unclear what caused the batteries to fail, but when it announced plans to ground U.S.-based 787s, the FAA said both battery failures released flammable chemicals, heat damage and smoke - all of which could affect critical systems on the plane and spark a fire in the electrical compartment.
The FAA has said it will keep the 787s grounded until airlines demonstrate that the battery system is safe and complies with safety regulations.