Extensive testing on lithium ion batteries during the 787 certification process led Boeing to believe that “significant overcharging” was the only occurrence that would cause the batteries to vent fire, according to a senior Boeing executive.

Testifying Tuesday in Washington DC at a US National Transportation Safety Board (NTSB) hearing, Boeing VP and 787 chief project engineer Mike Sinnett said testing done in 2006 and 2007 “used what was the state of the art at the time” to determine what could potentially cause the battery to fail and vent fire. Using a “nail penetration test” to intentionally cause damage to the lithium ion battery, “the only time we were able to make a [battery] cell vent with fire was significant overcharging,” Sinnett said.

NTSB is holding a hearing on the Jan. 7 Japan Airlines 787 lithium ion battery fire at Boston Logan Airport. The hearing’s primary focus is on the original certification process for lithium ion batteries for use on the 787, during which FAA issued “special conditions” since such batteries were “novel” on a commercial aircraft. “We are here to understand why the 787 experienced unexpected battery failures following a design program led by one of the world’s leading manufacturers and a certification process [conducted by FAA] that is well-respected throughout the international aviation community,” NTSB chairman Deborah Hersman said at the start of the hearing.

The 787 lithium ion battery fire in Boston was not the result of overcharging, NTSB and Sinnett have said. The Jan. 16 All Nippon Airways 787 lithium ion battery failure and overheating, which resulted in an emergency landing, was also not the result of overcharging, the Japan Transport Safety Board has concluded.

Though NTSB has not established a root cause for the Japan Airlines 787 fire, it has determined that short circuiting in a cell of the lithium ion battery that starts the aircraft’s auxiliary power unit led to thermal runaway—uncontrolled chemical reactions resulting from overheating—that cascaded to other battery cells and led to a fire.

Sinnett said that Boeing’s original lithium ion battery certification testing led it to believe that an “internal short circuit in [a single] cell” would not create “sufficient energy to propagate to other cells.” He said the “tests were rigorous” but conceded that “in retrospect [the testing] wasn’t conservative enough … We believed that the state of the art [testing] at the time was sufficient.”

Since overcharging was the main concern, Boeing developed “four layers of protection against overcharging” in the original 787 lithium ion battery design, Sinnett said. “FAA was closely involved with us for the entire process,” he noted.

FAA last week approved Boeing’s revamped design for the 787’s lithium ion batteries, paving the way for the lifting of the 787 grounding imposed by FAA Jan. 16.