As Ethiopian Airlines, Qatar Airways and other Boeing 787 customers are returning their Dreamliners to service with battery system modification kits, the U.S. National Transportation Safety Board (NTSB) is still looking for the cause of the January 7 APU battery fire aboard a Japan Airlines 787 parked at Boston Logan International Airport. The situation remains paradoxical–the FAA has certified the kit, thanks to the safety barriers Boeing (Chalet A324) has added, but the NTSB is still investigating. At stake is efficient use of lithium-ion batteries, a technology that brings performance and weight improvements in an era when electric systems are replacing hydraulic and pneumatic ones.
So what are the benefits of lithium-ion for batteries? “These batteries have lower weight and higher voltage, 3 to 4 volts, versus 1 to 2 volts for others,” Stanley Whittingham, a professor at Binghamton University in New York, told AIN. Whittingham was speaking at a forum the NTSB organized in April, about lithium-ion batteries in transportation. He pointed out the technology is more proven than one might think. For example, BAE Systems buses have accumulated 25 million miles in the U.S. using 11 kWh Li-ion batteries.
However, “although the chemistry is one that can provide very high energy density for rechargeable systems (a significant weight advantage for NASA), it is not the safest,” Judith Jeevarajan, a battery expert at NASA Johnson Space Center’s engineering directorate, stated. The January 7 fire, which sparked the grounding of the world’s entire fleet of 787s, was one of the most serious safety issues the large commercial aircraft industry has had to face in decades. “While we do not know the cause of the JAL battery fire, within a month our forensic work identified the origin of the event: short circuits in [battery] cell number six that cascaded, in a thermal runaway, to other cells,” NTSB chairman Deborah Hersman said during a later hearing.
The temperature inside the battery case exceeded 500 degrees F (260 degrees C). A simple short circuit inside a battery (whether it be lithium-ion or another technology) can easily degenerate into a catastrophic event. A cell contains both a fuel and its oxidizer–chemistry that is right for explosives and rockets.
The short circuit might have been caused by a dendrite, a small lithium outgrowth that can form inside a cell. It ends up piercing the polymer separator, creating a contact between the anode and the cathode–a short circuit. According to a source familiar with lithium-ion technology, repeated charges at cold temperatures, below zero degrees C, can help dendrites to form. The battery management system (BMS, or battery monitoring unit, in Boeing’s literature) is supposed to prevent charging below 0 degrees C.
While industry observers have often focused on the battery itself, its materials and so forth, the BMS plays a key role in safety. In addition to the aforementioned task, it has to prevent overcharge. Overcharging corresponds to exceeding the maximum voltage.
Each cell is managed individually. While the battery is charging, if a cell builds up a higher voltage than another one (such differences can stem from the aging process or manufacturing variations), the BMS discharges it to an average level. Then, it resumes charging. Overcharging a lithium-ion cell can cause a violent exothermic reaction.
The mission of the BMS is thus critical–so critical that the battery and the BMS are never really off duty. The voltage balancing function may work even though the aircraft’s master switch is in the “off” position.
Designers of lithium-ion battery systems have a hard time in testing. “There are important differences between safety abuse testing, versus field failures,” Daniel Doughty, president of Albuquerque, New Mexico-based Battery Safety Consulting Inc., said at the forum. Abuse tolerance is common to all cells, while, a field failure is a one-in-ten-million matter.
Moreover, in abuse tolerance testing, time constants are relatively long. In a field failure, “much higher temperatures can occur quickly,” Doughty said. An internal short circuit has fast kinetics for heat and gas generation.
A variety of events can trigger a thermal runaway. Most of them can be managed, according to Doughty. For example, the BMS manages (or rather prevents) overcharge. However, an internal short-circuit (another possible trigger) can’t. And the propagation of a thermal runaway can be managed in only a few cases. New technologies are needed to improve safety, Doughty emphasized.
NASA’s Jeevarajan insisted Li-ion battery designs should have high-fidelity thermal analysis. This shows that the battery design is safe under worst-case conditions. In addition, good thermal design extends the life of the battery.
Another way to progress may lie in the cathode material. West Covina, California-based battery manufacturer Concorde Battery, which had been a proponent of lead-acid technology, is now developing lithium-ion aircraft batteries. It is using “the safest chemistry so far developed for lithium-ion technology.” This features a cathode material of lithium iron phosphate, which inhibits oxygen generation, one of the main causes of fire in lithium-ion batteries. The cathode is thus no longer an oxidizer for a neighboring fuel. When Japan’s GS Yuasa designed a lithium cobalt oxide cathode for the 787 in the mid-2000s, the iron phosphate alternative wasn’t well developed. Safer alternatives are already available.