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Mark Verbrugge, a battery expert at GM’s research and development center in Warren, MI, says that such safety measures should be enough to keep batteries safe. Yet one factor remains outside the direct control of automakers. “The one thing that really worries OEMs is you can’t control poor-quality manufacturing as it relates to safety,” he says. For example, says Verbrugge, if “two electrodes touch because it’s poorly manufactured, you’ve got a problem.”

Such an internal short circuit can start an uncontrolled chemical reaction, Sandia’s Doughty says, adding that “if there’s a flaw in the manufacturing, and it has an internal short circuit, there’s nothing you’re going to do externally to interrupt that reaction.” Such problems are rare, occurring in one of ten million cells in laptops and other electronics, Doughty says. But, he says, “If there are 7,000 cells, and there’s one in ten million failures, you do the math in terms of how many vehicles are going to have a cell problem.”

Even if a bad battery does make it into a vehicle, however, it might not be a big problem. “We’ve designed it so that if you reach into our battery pack and deliberately set one of the batteries on fire, it doesn’t propagate to the neighboring cells,” says Eberhard. He gives two reasons: each battery comes in a steel case and a liquid cooling system can carry away the excess heat.

Yet Doughty says that during tests he’s seen violent explosions of the type that could potentially rupture a steel case. If the liquid cooling also fails, a single battery could cause neighboring batteries to overheat, setting off a cascade of small explosions. Doughty says electric car companies may be able to engineer systems that are “acceptably safe,” but he notes that, although “engineers always have multiple layers of safety, the worst accidents happen when, because of a very rare event, two or more of these multiple layers are compromised.”

Even if such an accident is rare, there could still be a backlash against electric vehicles. In particular, Doughty is concerned about conversion kits for turning conventional vehicles or hybrids in lithium-ion-based electric cars (see “Plug-In Hybrids Are on the Way”). “The thing I worry about is that one of these days there’s going to be a lithium-ion-powered vehicle that’s going to have a pretty spectacular accident – and then what are people going to say?”

Experts have two key recommendations for moving forward and producing even safer electric vehicles. First, automakers need to have a very strict screening process for their battery manufacturers, Verbrugge says. And, in the long run, Doughty says, it will be important to support research into new high-energy lithium-ion battery chemistries that are not prone to overheating.

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