雅思阅读复习资料(7)

　　The lithium-ion cells used in the Volt's battery pack have many virtues. They are much lighter and operate at a higher voltage than other rechargeable cells—and can therefore store more energy for a given weight. In addition, they have no "memory effect" (the tendency to accept less and less charge each time they are recharged) and can also hold their charge far longer than, say, the nickel-metal hydride cells used in the Prius. For good reason, all plug-in electric vehicles, including the Nissan Leaf and the forthcoming Ford Focus Electric plus Toyota's long-awaited plug-in Prius, have embraced lithium-ion chemistry.

　　But lithium is a highly reactive element. If overcharged, physically damaged or allowed to get too hot, lithium-ion cells can experience thermal "runaway" and even explode—as has happened on numerous occasions with the lithium-ion batteries in laptop computers and mobile phones. Also, if allowed to drain completely, they can short-circuit and make recharging dangerous. For these reasons, all lithium-ion rechargeable batteries contain circuitry that shuts them down when their voltage rises above or falls below a certain level.

　　To help keep the Volt's 435lb (197kg) battery pack at the right temperature, GM designed a sophisticated thermal-management system. This is separate from the main radiator system, which cools the range-extending motor-generator (a 1.4-litre petrol engine) and feeds the car's heater. The battery pack, mounted in a T-shaped steel tray with a plastic cover, runs down the centre of the vehicle.

　　GM believes the Volt's battery problem was caused by malfunctioning sensors rather than chemical reactions going haywire within the cells themselves. The company is currently developing fixes to make the battery's control systems sturdier. One proposal is to laminate the electrical circuitry. Another involves beefing up the cooling lines. A third is to reinforce the tray containing the battery modules.

　　Outsiders note that the lithium-ion pack in the Nissan Leaf—the only other mass-produced electric car currently on sale in the United States—is encased in a rigid steel box rather than a plastic framework. The Leaf has come through its crash-testing programme with flying colours. Interestingly, its battery pack manages without any additional cooling system.

　　Despite GM's experience with the ground-breaking EV1 electric vehicle in the 1990s, the company still has much to learn about the public-safety issues associated with powerful batteries. For instance, both GM and NHTSA kept their mouths shut about the Volt's initial fire for the best part of six months, claiming they needed time to assess the results and to carry out further tests. Others suspect they colluded to protect the Volt's fragile sales. GM hoped to sell a modest 10,000 Volts in its first year, but will be lucky to achieve even three-quarters of its goal.

　　In November, when GM finally went public about the Volt's fire problems, it warned owners, dealers and first-responders of the need to drain the car's battery pack after a crash. The OnStar communications system onboard every Volt should allow the company to dispatch an engineer to drain a battery anywhere in the country within 48 hours. For its part, NHTSA has now opened a formal safety investigation into the crash-worthiness of the Volt's battery system. Meanwhile, a congressional committee that oversees NHTSA is to hold hearings early in the new year to find out why it took nearly six months for the matter to be made public, and why the committee was not kept informed.

　　What is left unsaid in all this is the fact that conventional cars with a tank full of petrol are far greater fire hazards than electric cars will ever be. Some 185,000 vehicles catch fire in America each year, with no fewer than 285 people dying as a consequence. But, then, people have been living with the hazard of petrol for over a century. Irrationally, electric-vehicle fires are perceived as somehow more worrisome simply because they are new.