Controlling powerful battery's volatility poses urgent design challenge for Boeing
While the full fallout from the grounding of the fleet following two separate battery failures remains as yet unknown, there is at least one aspect over which there can be little argument.
The basic physics of the lithium-ion battery at the center of the 787 investigation cannot be changed, and the focus of the investigation has already shifted rapidly from whether the safety precautions in's design were sufficiently adequate, to more urgent questions over how quickly they can be modified.
Spurred on by the U.S. airworthiness authority's emergency directive, theprobe and the broader -led report will determine with Boeing what modifications are required to the battery-related aspects of the electrical system as well as whether the fire containment or protection system can—and should be—augmented. In the most extreme scenario for Boeing, this could conceivably lead to a change to alternate batteries, a new test effort, certification and modification program.
Until now, Boeing has remained unequivocal over the question of adopting or even studying different battery technology, saying simply: “We have no such plans at this time.” Outwardly, at least, the company remains confident in its choice of technology, which was driven by the lithium-ion battery's high power and energy density and its low maintenance requirements and low installed weight.
Yet the worldwide groundings, combined with NTSB images of the charred remains of the battery from the Boston incident, add to a growing litany of industry and public unease over the use of lithium-ion technology in aircraft. Even carrying lithium batteries as air cargo has proved lethal and prompted the International Civil Aviation Organization (ICAO) to issue stringent new rules governing their carriage as recently as Jan 1. Fires erupting in this type of battery, carried as cargo, were prime suspects in separate accidents involving twofreighters and a DC-8.
According to FAA figures, not counting the recent events, lithium batteries make up almost 80% of the 33 instances in which batteries have ignited on aircraft since 2009., which introduced the CJ4 business jet in 2010 as the first aircraft to enter service with lithium-ion batteries, was forced to replace them with nickel-cadmium after a battery fire on an aircraft in 2011. As with the 787, the FAA had also allowed the CJ4 to be certificated under special conditions that included added safety precautions for use of the lithium-ion battery.
In the case of the 787, two 32-volt lithium-ion primary batteries provide power as key elements of the aircraft's more-electric architecture. The main battery, located forward in the electric/electronic (E/E) equipment bay below the cabin floor by the front passenger doors, provides power for aircraft start-up, ground operations such as refueling and towing, and acts as backup power for the electrically actuated brake system. It can also assist the second battery, located in the aft E/E bay, in starting up the auxiliary power unit (APU) and, in the event of a power failure, energizes essential flight instruments in the flight deck until the drop-down ram air turbine spools up.
The battery that caught fire on the787 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 an787 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-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.
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