SEATTLE -- Boston airport firefighters encountered sizzling liquid and a hissing, "exploding" battery when they entered the 787 at the center of a 2-month-long National Transportation Safety Board investigation, according to documents released Thursday.

The NTSB said Thursday that it plans two public hearings next month, one to explore lithium-ion battery technology in general and another to discuss the design and certification of the Boeing 787 battery system.

The safety agency announced the hearings as it released an interim factual report and 499 pages of related documents on its investigation of the Japan Airlines 787 fire at the Boston airport on Jan. 7.

Among the findings in the documents released Thursday:

--On the day of the Boston fire, the battery did not behave as Boeing or subcontractor Thales predicted.

The battery's power discharge was "not at the constant rate described by the Boeing or Thales documents and included large changes and reversals of power within short periods of time," according to the Airworthiness Group Chairman Report.

--Sitting on a rack above the battery that burned was a smaller lithium ion battery, also supplied by Japanese manufacturer GS Yuasa, that is used to provide emergency power for the jet's flight controls "for a minimum of 10 minutes when no other electrical power is available."

Investigators found the exterior of this battery had been "lightly scorched" by the fire below and noted that its case had openings at the corners.

--No heat damage was found to any primary airplane structure.

However, the floor panel and carbon fiber floor support material, which are considered to be secondary structure, "were found to be heat damaged beneath where the APU battery had been installed."

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