The battery that caught fire on the Japan Airlines 787 in Boston was the second main battery. This unit's primary purpose is to electrically start the APU when neither of the engines is running and the aircraft is not connected to external ground power. In this case, the battery energizes the righthand of the two starter/generators connected to the APU. The aft battery also provides another minor role, namely to power navigation lights during battery-only towing operations.
The unit in the second incident, which forced an ANA 787 to make an emergency landing in Japan on Jan. 16, involved the main battery in the forward E/E bay. In this case, there was less damage, though spilled electrolytes, fumes and minor thermal damage indicated signs of overheating.
Mike Sinnett, 787 vice president and chief project engineer, says the lithium-ion battery has “the right chemistry it takes to have a large amount of energy in a short time to do the APU start, and allows us to recharge that in a short amount of time.” These qualities, added to the low weight, were sufficient to swing Boeing in favor of the technology in 2005, when it awarded the battery contract to Japan-based battery manufacturer, GS Yuasa, as part of the Thales-supplied electrical power conversion system.
The 787 contract marked the first commercial aviation application of lithium-ion technology and was selected over contemporary nickel-cadmium because it provided 100% greater energy storage capacity and double the energy from the same-sized unit.
“Lithium-ion wasn't the only choice, but it was the right choice for us at the time,” says Sinnett.
However, Boeing knew the outstanding performance of lithium-ion technology comes at a cost, namely development of an elaborate series of safeguards to prevent the battery from catching fire.
The GS Yuasa unit and its charging system is designed to modulate and control the energy flow so that over-charging, one of the identified causes of lithium-ion battery fires, cannot occur. Similarly, Boeing developed safeguards both inside and outside the battery to prevent it from over-discharging, or over-heating, both triggers for fires. However, in the worst-case scenario, which is known as a thermal runaway, once the reaction begins, there is very little to be done. As Sinnett comments, “fire suppressants just won't work. It's very difficult to put out with suppressants and you just have to assume its going to go.”
The problem lies with the lithium at the heart of the battery. Although this has twice the electrochemical potential of other materials, it also melts at a much lower temperature than other battery fuels, such as nickel. Energy experts in contact with Aviation Week say lithium melts at 357F—versus 2,800F for nickel—and acts “like molten sodium” in the process.
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.