Wednesday, November 6, 2013

Should Battery Fires Drive Electric Cars Off the Road?

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. (Tesla declined to provide someone to comment for this story but referred this reporter to the National Fire Protection Association and the company's online safety video.) 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.


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