In a February 7 news conference, NTSB chairman Deborah Hersman explained the latest findings on the battery problem that resulted in the grounding of the Boeing 787 fleet three weeks ago.

“After an exhaustive examination of the JAL [Japan Air Lines] lithium-ion battery, which was composed of eight individual cells, investigators determined that the majority of evidence from the flight data recorder and both thermal and mechanical damage pointed to an initiating event in a single cell,” she said. “That cell showed multiple signs of short circuiting, leading to a thermal runaway condition, which then cascaded to other cells. Charred battery components indicated that the temperature inside the battery case exceeded 500 degrees Fahrenheit.”

The cause of the short circuit is still unknown, although investigators have ruled out some typical lithium-ion battery faults such as physical damage to the unit itself and circuit shorts initiated by an external source.

The NTSB is still considering potential overcharging of the units, as well as possible manufacturing faults.