Need For Power
Mike Sinnett, 787 VP and chief project engineer, says the li-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 li-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.
“Li-ion wasn’t the only choice, but it was the right choice for us at the time,” said Sinnett.
However Boeing knew the outstanding performance of li-ion technology comes at a cost, namely development of an elaborate series of safeguards to prevent the battery catching fire.
The GS Yuasa unit and its charging system are designed to modulate and control the energy flow, so that over-charging, one of the identified causes of li-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 says, “Fire suppressants just won’t work. It’s very difficult to put out with suppressants and you just have to assume it’s 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 much lower temperature than other battery fuels, such as nickel. Energy experts tell Aviation Week lithium melts at 357 degrees F versus 2,800 degrees F for nickel, and acts “like molten sodium” in the process.
The other issue associated with the battery design is that to generate the high energy density the unit is made up of a stack of tightly packed cells. 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 be in the case of 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.”