雅思阅读材料：Difference Engine

　　GM claims the initial fire in June would never have happened if the NHTSA’s engineers had drained the Volt’s battery immediately after the impact. It is odd that they did not. When crash testing a conventional petrol-powered car, the standard procedure is to drain the fuel tank to prevent any chance of fire. It would seem reasonable to do the equivalent with an electric vehicle.

　　But, then, GM did not adopt a "depowering" protocol for the Volt until after the June fire. Even when it did, it failed to share the procedure with the safety agency until embarking on the November tests. In the wake of the latest findings, GM is now working with the Society of Automotive Engineers, NHTSA and other vehicle manufacturers, as well as fire-fighters, tow-truck operators and salvage crew, to implement an industry-wide standard for handling battery-powered vehicles involved in accidents.

　　Toyota ran into similar troubles when its Prius hybrid car was introduced over a decade ago. Though the Prius’s battery pack is considerably smaller than the Volt’s, fire-fighters and other first-responders had to learn how to disarm the vehicle following an accident—by removing fuses from under the bonnet and pulling a catch beneath the rear storage area to isolate the high-voltage system. Until they had done so, they were warned, they were on no account to take a metal cutter to an overturned Prius to extricate trapped occupants. Lurking beneath the floor was a big orange cable carrying a heavy current that would have fried anyone slicing through it.

　　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.