The battery unit sustained severe fire damage, the safety board said, adding that it had sent two additional investigators to Boston.
Japan's ministry of land and transportation ordered inspections of the same batteries in all Boeing 787s. So far, no irregularities have been found, but the results will be shared with U.S. authorities, the ministry said.
United Airlines, a Dreamliner operator, inspected its six 787s following the Boston fire as a precaution, but would not comment further.
Boeing said the fire appeared unrelated to previous problems involving 787 electrical power systems and that it was cooperating with the safety board.
On Wednesday All Nippon Airways canceled a domestic flight in Japan because of an error message on the braking system of a 787. An airline official said it was not a mechanical problem, but a computer error on the electric brake system controls. Passengers on the flight were moved to a later flight and the computer part will be replaced, an ANA official said.
In previous incidents, one of the 787 test flight aircraft lost primary electrical power in 2010 and was forced to make an emergency landing in Texas. All aboard evacuated safely.
An engine failed during tests on the ground in South Carolina last July and inspectors found a similar problem on another aircraft in September.
In December, another new 787 operated by United diverted safely to New Orleans after experiencing mechanical problems.
Some safety experts are concerned, but not alarmed, about the mechanical setbacks with the Dreamliner since its delivery to airlines beginning in 2011, following years of manufacturing delays and cost overruns.
They say new aircraft models often have "growing pains." Other experts have said any Dreamliner service problems would be magnified because of its problematic history during development.
As the first commercial jetliner built mostly from lightweight carbon fiber, the twin-engine Dreamliner has been touted for fuel efficiency in an era of soaring fuel prices. It has attracted enormous interest from airlines, with most orders coming from overseas carriers initially.
Boeing shares were off sharply on Wall Street relating to its Dreamliner problems this week.