Lithium-ion For Airplanes: No Thanks
Now that the world is once again swimming in oil and gagged with shale gas, it's hard to imagine that in 1976, no less an august energy player than Exxon thought the world was running out of oil. So it wanted to get into the next big energy thing, which would be rechargeable lithium batteries for electric cars. It launched a well-funded research project and developed a promising design which, according to Seth Fletcher's Bottled Lightning: Superbatteries, Electric Cars and the New Lithium Economy had one big problem: When they sent a sales rep to Chicago to show the batteries, Exxon discovered that the electrolyte was breaking down into a gas that would ignite on contact with air. "So each day, Exxon's man in Chicago would show off the company's breakthrough...then each night in his hotel room, he would carefully twist off the top of each battery and watch as a fireball leaped out." Exxon decided to stick with oil and gas.
Forty years later, lithium batteries are thought to be on the verge of a demand surge, driven by equally strong demand for electric cars. And as sure as cars will be lithium-battery powered, so will airplanes—both as primary power and for starting batteries. It's already happening, in fact. But the ghosts of Exxon's fireballs still haunt the lithium battery industry, even though modern versions are more stable than Exxon's lab models could ever have been. Some aviation battery manufacturers—Concorde is one—are reluctant to aggressively offer lithium-ion for safety and cost reasons, but mostly safety. The FAA has expressed reservations about lithium-ion technology in airplanes and apparently with good reason.
Obviously, the agency has stringent requirements for certification of aircraft lithium batteries, but the industry was surprised to learn that this wasn't enough to prevent a lithium-ion battery from lighting off in a Cessna Citation CJ4 last year. Neither Cessna nor the makers of the cells, A123, would comment on what happened to cause that fire, but Cessna quickly withdrew the batteries from the Citations and the FAA issued an emergency AD. Cessna says it's still committed to Li-ion, but declines to offer details. Ironically, A123's cell technology—lithium-ion nanophosphate—is considered the most stable of the principle lithium chemistries, yet it still notched an aircraft hull fire.
The risk of lithium-ion battery explosions and fires, while low, is complicated by their potential severity. Once it ignites due to thermal runaway, a lithium battery fire is difficult if not impossible to extinguish and it will often lead to a chain reaction, igniting neighboring cells. For airplanes, the nature of the risk is bifurcated. For starting batteries which must be charged in flight, an overcharge/imbalance in one cell can lead to thermal runaway. Li-ion technology is equipped with electronic protection to prevent this and also to keep the batteries from discharging to zero, which will trash them. Primary electric airplanes don't have much risk here, since they're charged on the ground, not in flight.
The second risk relates to cell shorts which, no matter how good the quality control, are thought to occur once in every one to five million cells. A short can lead to thermal runaway. That sounds like a small risk and probably is. But consider this: UPS has suffered two aircraft hull-loss fires in airplanes carrying lithium-ion batteries. Although investigators couldn't pin the source of the fires on the batteries, they were prominently mentioned in the accident reports. One of the airplanes was carrying 81,000 Li-ion batteries.
Knowing what I know about Li-ion, I'm not interested in having one in an airplane I fly, thanks. The technology will get there, I'm sure, but I'm perfectly happy to wait until it's more stable and robust. What's the rush? (I came to this conclusion after one lithium battery expert I spoke to said he found the power outlets in airliners being used to run and charge laptops to be a really bad idea.)
As you can surmise, however, aviation won't drive much primary development in Li-ion. OEMs will figure out better electronics and safety containment systems, but they'll use cells from A123 and other major providers. In fact, that's just what Mid-Continent Instruments and Avionics has just announced it plans to do.
What will push Li-ion technology is the electric vehicle market, which is the next big industrial crap shoot. Companies like A123 are banking on rapid uptake of EVs, this despite a long history of failure of electric cars to compete with internal combustion engines. "This time it will be different," goes the mantra. Maybe so. But see above. Exxon thought the end of the age of oil was in sight 40 years ago, but we have more producible oil and gas than ever. If gas reserves keep growing exponentially and if gas-to-liquid becomes economically practical, electric cars—and the batteries to power them—may have another tough time of it.
In a future blog, I'll examine hybrid technology for aircraft. It's coming, too.