The other issue associated with the battery design is that the unit is made up of a stack of tightly packed cells to generate the high energy density. Each cell consists of a layer of lithium, acting as the cathode, separated from an oxidizer, or anode, by a thin layer of ion-conductive polymer. If a short occurs, and the lithium melts, the lithium reacts first with the electrolyte and then the oxidizer before propagating to other cells. This process, which does not occur with nickel-cadmium or nickel-metal hydride batteries, is the “thermal runaway” circumstance cited by Boeing.
If the chain reaction starts, as is believed to have occurred in the Boston event, the current procedure in flight is to vent smoke overboard from the E/E bay. The energy release from the lithium, however, cannot be stopped and will only cease once the material has been consumed by the reaction. The initial NTSB investigation found that although the APU battery had been severely damaged by the fire, the thermal damage to the surrounding structure and components was “confined to the area immediately near the APU battery rack (within 20 in.) in the aft electronics bay.”
However, while this would appear to be good news in terms of containment, an update from the NTSB released on Jan. 14 indicates that Boston firefighters had been “able to contain the fire using a clean agent (Halotron),” suggesting that without their efforts the damage would almost certainly have been far worse. Halon fire suppression is provided in the cargo hold but not the E/E bay.
The NTSB investigative team includes subject-matter experts such as the U.S. Naval Surface Warfare Center's Carderock Div. in West Bethesda, Md. The Navy has bitter experience with the technology, having lost a prototype mini-submarine known as the Advanced SEAL Delivery System in 2008 because of a lithium-ion battery fire.
Get detailed data on ANA's and JAL's incident aircraft in the digital edition of AW&ST on leading tablets, or go to AviationWeek.com/787battery