|November 13, 2013|
Should Battery Fires Drive Electric Cars Off the Road?
|On October 1, Rob Carlson drove his Tesla Model S down Route 167 outside Seattle. He accidentally ran over a piece of metal, likely a fender or other curved piece of metal from a truck. That metal somehow punctured the quarter-inch thick armored undercarriage of the vehicle and penetrated its battery pack. Within 30 minutes, the car was in flames---the first fully electric vehicle fire on the road in the U.S. and a viral video sensation.|
Fortunately, the Model S comes equipped with a warning system. "The car warned the driver to get off the highway as soon as the incident happened. That's awesome," notes chemist Jeff Chamberlain, deputy director of the Joint Center for Energy Storage Research at Argonne National Laboratory, otherwise known as the U.S. government's battery research hub. "Of course, any $80,000 car should be able to do that for you."
A few weeks later, a driver in Merida, Mexico, lost control of his Model S at high speed, crashing through a concrete wall and into a tree. The driver and his passengers were able to walk away, apparently uninjured before the Tesla burst into flames. And now a third Model S has caught fire after an accident initiated by yet more road debris---this time a renegade tow hitch near Smyrna, Tenn., (which is coincidentally where Nissan builds its all-electric LEAF that uses similar lithium-ion batteries)---that again appears to have pierced the car's battery pack and set it ablaze on November 6.
Battery fires are not an issue confined to the Tesla Model S, which is by some measures the world's safest car. When doused with seawater after Superstorm Sandy, a fleet of 16 Fisker Karma's burned last October. And a Chevy Volt sitting in a garage weeks after safety testing by the National Highway and Traffic Safety Administration (NHTSA) suddenly began to smolder and burn in 2011, prompting a full safety investigation that later cleared the model for sale. That's 20 or so incidents in the past few years. For comparison, note that there is a fire in the predominant type of vehicle on the road---a car powered by an internal combustion engine vehicle---every four minutes or so.
Nonetheless, battery cars can burst into flames. This is not a problem confined to cars---think Boeing's Dreamliner or any number of Sony products. Rather, it is a problem confined to batteries. Pack a lot of chemical energy into a small space and if something goes wrong, fire or explosions are the inevitable result. So what should be done in the event of such novel fires?
Lithium-ion is the world's most popular battery technology, employed by the hundreds of millions in everything from cell phones to electric cars. Yet such mishaps have proved extremely rare.
Here's how a lithium ion battery works. A plastic film separates a positive and negative electrode, all of which is bathed in electrolyte, in this case a clear chemical solvent. The electrolyte is a carbonate liquid that ionizes the lithium, causing it to pick up an extra electron. Each of these lithium ions then acts as a shuttle of sorts, carrying that extra electron from the anode to the cathode. At the cathode, the lithium ions are absorbed, freeing up those ionizing electrons to act as current. To recharge the cell, simply add electricity, which drives the lithium back out of the cathode and into the anode, and it's ready to do it all over again.
Now tightly roll sheets of anode and cathode material and cram it into a cylinder. That's one lithium ion battery. All kinds of things can go wrong in this set up, from a build up of gas that bursts the exterior cylinder to an actual metallic lithium link forming between the anode and cathode that then sets off what engineers call "thermal runaway." It's more commonly known as fire, helped along by the fact that other components of the cell, such as the plastic separator and the organic solvent burn nicely, much like gasoline. "It's a chemical fire at its heart," Chamberlain explains.
Put enough cells next to each other and a defect in one can quickly become a defect in all, thermal runaway on the scale of a car-sized battery pack. These breakdowns of the battery generate their own heat, or, in the words of chemists, the reaction is exothermic---enough so that the heat from one cell can set off another. That's why the software to manage the cooling and recharging of electric vehicle batteries is as important as the lithium ion battery pack itself.
And that's where Tesla has distinguished itself. The new car company confines each Model S's more than 6,500 lithium ion batteries from Panasonic in 16 individual modules---separate but equal and comprising the vehicle's overall battery pack. By separating the modules in this way a mishap in one module is unlikely to spread to another module. In addition Tesla's battery pack is cooled with a glycol-based chemical cocktail, blue in appearance, that can quickly whisk away any excess heat. There is also a "firewall" between each module, according to Tesla CEO Elon Musk, suggesting that some kind of heat resistant material is segregating the modules. It seems that just one module burst into flames in the October 1 incident in Washington involving the pierced battery pack. The battery management system worked well enough that the car's navigation system warned the driver to pull over and get away from the vehicle. He walked away from the accident unharmed.
That is often not true of crashes involving gasoline.
And keep in mind that part of the reason an electric vehicle cannot go as far as a gasoline-burning car is that even the best lithium ion battery only holds roughly 200 watt-hours of energy per kilogram. Gasoline holds 1700 watt-hours per kilogram. Less energy stored means a reduced risk of that energy unleashed. "We are already carrying around really energy dense materials in our vehicles," Chamberlain notes. "We should be comfortable."
Or as Tesla's Musk put it: "For consumers concerned about fire risk, there should be absolutely zero doubt that it is safer to power a car with a battery than a large tank of a highly flammable liquid."
Put out the fire
Once a battery fire gets started, however, that fire can be harder to put out than a gasoline fire. In Washington, firefighters did not help matters by cutting holes into the metal frame and thus allowing more oxygen to reach the battery fire in progress. The best thing to do may be nothing.
"In our controlled laboratory setting, we prefer to let a lithium-ion battery fire burn itself out," says Chris Orendorff, principal investigator for the Battery Abuse Testing Laboratory at Sandia National Laboratories, or the guy whose job includes finding out what it takes to blow up any given battery. That is also the guidance that Tesla gives first responders in its emergency response guide: "Battery fires can take up to 24 hours to fully extinguish. Consider allowing the vehicle to burn while protecting exposures."
If a more aggressive course of action is taken, as also happened in the Washington State wreck, beware of putting too little water on a lithium-ion battery fire. If the amount of water is insufficient, the fire will simply appear to go out---before bursting out anew. And if small amounts of metallic lithium have formed as a result of the lithium-ion failure (lithium-ion batteries, despite the name, typically do not contain metallic lithium), the reactive metal can burst into flames because of something as simple as humidity in the air. In attempting to douse a lithium-ion fire, either a lot of water is required or alternative fire suppressants, like CO2 or other chemicals such as Halon. The National Fire Protection Association notes this as an area requiring more research to determine the best approach.
More worryingly, if left to itself after being damaged in an accident, a lithium-ion battery can slowly degrade to the point where it spontaneously bursts into flames weeks later. In essence, if damaged a lithium-ion battery can produce hydrofluoric acid (one of the most powerful acids on Earth) from fluorinated compounds in the battery that then further damages the cell itself and potentially allows the conditions to become right for a fire. Something similar is what happened in the case of the Chevy Volt that burst into flames weeks after safety testing.
Then there's the toxic vapors: the aforementioned sulfuric acid, plus bits of various metals that can be liberated by the fire, including aluminum, cobalt, copper, lithium, and nickel. Anyone anywhere near such fires, particularly in an enclosed space, should wear full protective gear and self-contained breathing apparatus that allows no outside air into the system. Sulfuric acid is no picnic (although it also finds use in the electrolyte of some lead-acid batteries and is part of the reason that more than 2,000 people suffer chemical burns from using lead-acid batteries, such as the ones in conventional cars, each year.)
Already, more than 100,000 electric cars ply U.S. roadways. Such novel fires will become more common as more and more electric vehicles hit the road, whether the luxury Tesla Model S or cheaper alternatives such as the Ford Fusion Energi. That will in turn mean that the NHTSA and other regulators will need to devise new e-car safety tests, a process that an interagency task force, including the U.S. Department of Energy is currently working to complete.
One reason that Tesla burst into flames in Washington is inherent to the design itself. The battery pack comprises the very bottom of the Model S. This "skateboard" design is what makes the Model S so stable to drive.
But the skateboard design also exposes the battery pack to the hazards of the road. A piece of metal run over by a gasoline-powered car would have torn up the muffler and exhaust system, or possibly punctured a fuel line. In the case of the Tesla Model S, it punctured the battery and set off a fire. The NHTSA will investigate. "It is important to evaluate and understand a cell or battery response to all types of abusive scenarios," Sandia's Orendorff argues, "so that proper design, chemistry or engineering improvements can be made to mitigate these risks."
Or, as Carlson put it in an email to Tesla customer service: "I was thinking this was bound to happen, just not to me. But now it is out there and probably gets a sigh of relief as a test and risk issue---this 'doomsday' event has now been tested, and the design and engineering works." In other words, battery fires are no reason to kill the electric car.