Short Circuit Found In Fire Damaged 787 Battery

Wired:

The National Transportation Safety Board believes an internal short circuit within a single cell inside a lithium-ion battery led to a fire aboard a Boeing 787, shedding new light on the battery problem that has grounded every one of the 50 Dreamliners in service worldwide.
The agency said Wednesday that it has completed disassembling the 32-volt battery that caught fire on a Japan Airlines 787 after passengers had disembarked in Boston on January 7. Investigators found evidence that the fire — called “thermal runaway” — started with a short circuit in cell no 6. There are eight cells in the 63-pound lithium-ion battery, and the NTSB said it found evidence that cell no. 6 sustained multiple short circuits. Investigators have ruled out mechanical damage as a cause of the short, as well as the possibility that the short circuit occurred between the cell and the battery case. Rather, the damage to the case containing the battery was caused by the fire that resulted from the short.
“The short circuit came first, the thermal runaway followed in cell no. 6 and it propagated to the other cells,” NTSB chairman Deborah Hersman told reporters in a press conference this morning. Hersman said they have yet to find the cause of the short circuit but are looking at several possibilities.
“We are looking at the state of charge of the battery cells, we are looking at manufacturing processes and we are looking at the design of the battery,” she said.
The new information came the same day that Boeing flew a 787 from its paint facility in Texas back to its factory north of Seattle. The flight was a one-time-only ferry flight approved by the Federal Aviation Administration, and only the pilots were allowed aboard. They closely monitored the battery during the flight to Paine Field and experienced no problems during the three-and-a-half-hour trip. It was the first flight of a 787 since the entire fleet of Dreamliners was grounded pending an investigation by the FAA.

At least they have something now.  Paraphrasing Donald Rumsfeld, a known unknown is better than an unknown unknown.