Japan Transport Safety Board chairman Norihiro Goto told reporters the jet's data recorder showed the main battery, used to power many electrical systems on the jet, did not exceed its maximum voltage. That contradicts an earlier assertion by the agency as it investigates with the U.S. Federal Aviation Administration.
All 50 of the 787 Dreamliners that Boeing has delivered to airlines were grounded after the emergency landing by the ANA flight in western Japan on Jan. 16. Boeing has halted deliveries of new planes until it can address the electrical problems.
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 may have gone wrong.
"It's not that it is difficult, but that we are not so familiar with it," Goto said.
The Transport Safety Board said it also will 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.
U.S. investigators also said that they found no evidence of overcharging in a battery that ignited on a Japan Airlines Boeing 787 as it sat on the tarmac in Boston's airport earlier this month.
US officials defend handling of Boeing 787 mishaps
Meanwhile, Obama administration officials are struggling to defend their initial statements that the Boeing 787 is safe. They are promising a transparent probe of mishaps involving the aircraft's batteries.
Transportation Secretary Ray LaHood stood by his Jan. 11 assertion that the 787, Boeing's newest and most technologically advanced airliner, was safe. At that time, he and the head of the Federal Aviation Administration, Michael Huerta, declared the plane fit to fly despite a battery fire in one plane.
Five days later, following another 787 battery mishap in Japan, LaHood and Huerta ordered the lone U.S. carrier with 787s to ground the planes. Authorities in other countries swiftly followed suit.
Huerta, joining LaHood, said FAA is working as quickly as possible to find the cause of the problems.