The risk of fire from overheating powerpacks emerged as a major concern for Boeing's cutting-edge new planes after pilots were forced to land a domestic Japanese flight due to smoke apparently linked to the lithium-ion battery.
The planes suffered a series of glitches earlier this month, prompting airlines to ground all 50 of the world's operational 787s after a global alert issued by the US Federal Aviation Administration.
But the National Transportation Safety Board said its examination so far has shown the battery was not the culprit of an earlier January 7 fire on an empty Japan Airlines plane in Boston.
"Examination of the flight recorder data from the JAL B-787 airplane indicate that the APU battery did not exceed its designed voltage of 32 volts," a statement said.
The physical examination of the battery, including x-rays and scans of the assembled battery and of its disassembled components, was still ongoing, the agency said.
The NTSB added that representatives from its Japanese and French counterparts were participating in the investigation, and noted that it had sent its own investigator to Japan for the investigation of the incident there.
On Friday, Boeing announced it was halting deliveries of its 787 Dreamliner, but said it would continue to build the aircraft while safety experts examine its battery and electrical systems.
The problems have cast a cloud over the aircraft, which is heavily dependent on pioneering electrical systems and lightweight composite materials and is seen as key to Boeing's future.
No airline has cancelled purchases for the 787, but with 850 of the ambitious $200 million-plus aircraft on order, a fortune is at stake.