Tokyo - This summer marks a milestone in green motoring. Tokyo-based Mitsubishi Motor will soon start selling its $48,000 electric minicar, the i-MiEV, beating out Subaru (FUJHY), Nissan Motor (NSANY), and China's BYD Auto, all of which have designs on the electric-car market.

Yet even as carmakers race to showcase these green vehicles, some experts are raising concerns. They fret that short circuits—or damage from a crash—could cause the latest generation of batteries using lithium-ion technology to overheat. In mobile phones, laptops, and other gadgets, that happens in 1 of every 5 million to 10 million "cells" that make up the battery, which means fires are quite rare. But while a laptop battery usually consists of a half-dozen cells, electric cars will need at least 75 or 80, which means one in every 60,000 electric cars hurtling down the highway could have a problem. "It's not going to happen all that frequently, but the consequences could be catastrophic," says Brian Barnett, a battery expert at Tiax, a technology development company in Cambridge, Mass.

One question is whether electric cars will be any less safe than a vehicle filled with 10 to 20 gallons of gasoline. With this kind of battery technology still in its infancy, it's hard to quantify the danger, but it may not be negligible. "A significant risk is that one cell failure will take other cells with it and then [the trouble] could spread," says Menahem Anderman, president of Advanced Automotive Batteries, a consulting company in California. Even if battery fires were no more widespread than those in gasoline cars, the headlines could stall any transition to electric vehicles.


Experts say defects in batteries might not crop up right away. That means thousands of electric vehicles could be on the road before the first incident occurred. Even if there were no injuries, the car companies would suffer a blow. After Sony (SNE) recalled faulty Vaio laptops in 2006, it booked a charge of $258 million. In electric cars, where batteries account for about half the cost of the vehicle, battery failures could prove far more costly.

Manufacturers insist their cars will be safe. Nissan says it coats its batteries in a special material that minimizes the risk of overheating and isolates cells that get too hot. In the Mitsubishi i-MiEV, the batteries are protected by both the car's rigid skeleton and a second anti-impact cage. Chips will control charging and will continuously monitor the performance of each cell. Considering all these protections, gasoline is "far more dangerous" than batteries, says Osamu Masuko, president of Mitsubishi Motors, who has been using a red-and-white chauffeur-driven i-MiEV for nearly two years without a hitch.

To ensure safety, carmakers are rigorously testing their batteries. Nissan and Mitsubishi, for instance, expose them to extreme heat and cold, charge and discharge them continuously, and crush them to simulate crashes. "We had to think of every situation we could imagine," says Hideaki Horie, who heads Nissan's battery group. And when Toyota (TM) markets its lithium-ion-powered plug-in Prius hybrids later this year, it will be on a lease basis so the company can keep tabs on the cars.

Such measures don't fully reassure concerned experts. "Until you put the batteries into a car and have someone drive it for a couple of years," says Seiji Ezaki, battery analyst with Tokyo market researcher Fuji-Keizai Group, "you won't know how the car will be used or how the battery will hold up under real-world driving conditions."

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