The forthcoming Chevy Volt and Nissan Leaf rely on lithium-ion battery packs, as do other contenders on the electric-car circuit. Yet perhaps mindful of a few highly publicized fires touched off by early lithium-ion power packs in laptops, are consumers assured that their car batteries will remain safe, even in an accident?
Much of the assurance falls under the purview of Sandia National Laboratories’ Battery Abuse Testing Laboratory, which has become the de facto automotive battery-testing shop in the U.S. The lab heats, shocks, punctures and crushes batteries to see how safe they would be in crashes and extreme operating conditions.
When lithium-ion cells first came to the laptop market, “the active materials were very energetic. There were some significant field failures,” notes Chris Orendorff, the battery lab’s team leader. The usual cause was thermal runaway, a chemical reaction that could start from excessive overheating, then potentially cause a cell to catch fire or explode. Although even extreme driving conditions are unlikely to trigger those problems, a crash could, and so could a sudden overcharge—for example, if lightning struck a charging port while a car was being recharged.
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Small tweaks in chemistry can make a large difference in how well battery packs resist overheating or exploding. “Half a dozen different chemistries are still being considered as viable” in terms of performance and safety, Orendorff says. Sandia is seeing more designs with lithium iron phosphate cathodes, for example, because they stay cool and suffer little degradation over time. Additionally, batteries with anodes made from lithium titanate seem less likely to overheat even under hot driving conditions. Electrolytes containing different lithium salts are still being tested for greatest stability, too.
Manufacturers are also testing a variety of mechanical safety features similar to measures developed to prevent thermal runaway in laptop lithium batteries. Among them are vents that discharge gases that build up during errant reactions and current-interrupt devices that trip like circuit breakers to disable overly active cells.
After years of experimentation, progress in fabricating and testing advanced batteries for fuel-efficient vehicles has accelerated in part because millions of research dollars were appropriated under the 2009 American Recovery and Reinvestment Act. Sandia recently received $4.2 million of stimulus money for upgrades that will allow it to test more batteries and do it faster.
Of course, Sandia and the manufacturers want to prevent all possible dangers. But, Orendorff asserts, consumers forget that no car is completely hazard-proof. Lithium-ion batteries may have a higher chance of igniting than, say, standard lead-acid batteries, “but the chances of flammability are far less than what you have in a combustion vehicle that is carrying 15 gallons of gasoline onboard.”