Laptops equipped with lithium-ion batteries occasionally overheat and catch fire. This has some people concerned about the use of this type of battery in new electric sports cars and kits for converting conventional cars and hybrid vehicles into all-electric cars.
It’s an exciting time for electric vehicles – with regular announcements of increasing storage capacities for battery materials (see “Battery Breakthrough”) and exotic, high-priced vehicles slated to come onto the market, such as the recently announced sports car from Tesla Motors of San Carlos, CA. But electric vehicles have failed in the past. If they’re going to succeed this time around, they’ll need to win over the general consumer, and that will mean, among other things, demonstrating that the powerful battery packs are safe.
Lithium-ion batteries have long been favored for powering laptops and cell phones because they’re small and light. But packing so much energy into a small space is also dangerous. The batteries have been known to burst into flames, sometime violently; and because both the fuel and the oxidizer are bundled into the battery, they can’t be smothered like common fires, says Dan Doughty, who manages lithium-ion battery testing at Sandia National Laboratories in Albuquerque, NM.
The key safety challenges are preventing overcharging, overheating, and damage in an accident. In each case, chemical reactions can get out of control, causing “thermal runaway,” which can generate temperatures hot enough to melt aluminum and cause batteries to explode, he says.
According to the U.S. Consumer Products Safety Commission, from 2003 to 2005 more than 300 incidents occurred involving lithium-ion laptop and cell-phone batteries overheating or catching fire. Many of the incidents involved personal injury.
This potential problem with lithium-ion batteries is multiplied by the thousands in vehicles. In the case of Tesla Motors’ car, for example, almost 7,000 batteries are packed behind the passenger compartment to power the car (to an impressive 60 mph in about four seconds).
But the company has done a lot to keep its battery-powered system safe – much more than is done in laptops, says CEO Martin Eberhard.
To keep temperatures under control, Tesla’s engineers have developed an electronically controlled liquid cooling system. They have also included overcharging protection, three layers of fuses, and sensors that will trigger the batteries to disconnect in the case of high-temperatures, a sudden impact, or a roll-over. In fact, the decision to use many small batteries rather than a few very large ones was in part a safety consideration – each battery and its relatively small amount of stored energy compared with the entire system is isolated and protected within its own steel case. And the entire system is also encased for protection in the case of an accident.
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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