Are Lithium-Ion Electric Cars Safe?

The use of a type of battery with a history of overheating raises safety concerns.

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.