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The second battery was of identical design but used for a different purpose than the first, the agency said. Its report Sunday added scattered details to what is known about the incident, one of two battery problems that led to the grounding of all 50 of the 787s in airline service.

The board said that its laboratory was continuing to study the destroyed battery, whose function was to start the auxiliary power unit, a small jet engine used mostly on the ground. The battery, which was not being charged or discharged, caught fire on Jan. 7 while the plane was empty.

The undamaged battery on which the board reported Sunday was a backup for cockpit instruments, located near the nose. The board has released photos of the damaged and undamaged batteries.

On Jan. 16, on a different 787 on a domestic flight in Japan, the main battery began belching smoke a few minutes after takeoff, forcing an emergency landing. Investigators have not said whether it was being charged at the time. The planes were grounded shortly after.

The batteries use a lithium-ion chemistry, which has been in use for many years in many applications but is new in airplanes. Investigators say the problem could be with the batteries or with the associated electronics used to manage them.

The board said Sunday that at the time of the fire at Logan, the plane, which belonged to Japan Airlines, had completed 22 flights and 169 flight hours.

The board's update also said that investigators had completed a review of two systems associated with the auxiliary power unit, at two locations in Arizona, and found no problems.

The board said it sent two additional investigators to Seattle, where the board is working with the Federal Aviation Administration to review work at Boeing. One of the investigators will work with a group reviewing Boeing's corrective actions, and the other will work on how the lithium-ion batteries were approved by the F.A.A. The safety board is an advisory body with no regulatory authority – that belongs to the F.A.A. — but it is in charge of safety investigations.

A team led by the safety board also examined circuit boards used to monitor the battery in the in-flight incident in Japan, the board said. The circuit boards were damaged in the incident, "which limited the information that could be obtained from tests," the board said. It added that the team "found no significant discoveries."