The U.S. National Transportation Safety Board chairman, Deborah Hersman, said yesterday that the battery that recently caught fire on a Boeing 787 Dreamliner in Boston showed signs of short circuiting and a phenomenon called thermal runaway.

The specific lithium ion battery chemistry used in the airplane—lithium cobalt oxide—is particularly prone to thermal runaway, in which heat in a battery triggers more heating until it catches fire (see “Grounded Boeing 787 Dreamliners Use Batteries Prone to Overheating”). Thermal runaway can be triggered by a short circuit between the electrodes in a battery. The NTSB said that it is not clear why safeguards put into place to stop thermal runaway didn’t work.

Short circuits can be the result of damage to the battery or manufacturing defects that pierce a polymer barrier between electrodes in lithium ion batteries (see the structure of a “Lithium-Ion Battery”). Metal particles from a defective manufacturing process led to overheating in Apple and Dell products several years ago, resulting in massive recalls (see “Lithium-Ion Batteries That Don’t Explode”).

Boeing says it can’t comment directly on the investigation or its choice of battery chemistries. While lithium cobalt oxide isn’t the safest chemistry, it’s one the industry has long experience with, and it stores more energy than safer chemistries.