Boeing's fleet of Dreamliners -- its newest and most advanced plane -- was grounded by the Federal Aviation Administration after a series of electrical fires related to its lithium-ion batteries.
The investigation team had already disassembled the main batteries in the plane that caught fire after passengers disembarked in Boston and one other that caught fire and forced an emergency landing in Japan. The NTSB determined the batteries had not exceeded their designed voltage of 32 volts.
The NTSB has moved on to taking apart the eight cells of the APU charging unit from the Boston plane in its materials laboratory in Washington, a release from the board said.
"Examinations of the cell elements with a scanning-electron microscope and energy-dispersive spectroscopy are ongoing," the release said.
"A cursory comparative exam has been conducted on the undamaged main battery. No obvious anomalies were found. More detailed examination will be conducted as the main battery undergoes a thorough tear down and test sequence series of non-destructive examinations," the NTSB said.