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07-17-07, 10:10 AM #1
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Electric Cars Nearly Ready, but Batteries Are Less So
CLEVER and attractive, the Chevrolet Volt, a design study for a new wrinkle in electric cars, dominated the headlines coming from the Detroit auto show in January. But the introduction was punctuated with an asterisk.
The car that promised a fuel economy equivalent of 150 miles a gallon and a total range of 640 miles using its onboard recharging system carried a major caveat: the lithium-ion batteries required to make it a reality are not yet available, and won’t be until 2010 at the earliest, industry experts say.
The Volt is not the only car waiting for lithium-ion batteries to be roadworthy. Reports last month in Nikkan Kogyo Shimbun, a Japanese business newspaper, said that the next generation of the Toyota Prius would be delayed by six months because the carmaker had decided that lithium-ion batteries were not quite ready.
Officially, the car was not postponed because Toyota had never announced an introduction date, but such a decision would have major implications: reverting to nickel-metal hydride batteries in today’s Prius means finding room for a larger and heavier power pack. A Toyota spokesman, John Hanson, said that while the company saw “huge potential” in lithium-ion batteries, it wanted to assure future Prius buyers the same levels of affordability and reliability they experience in today’s models.
The quest for batteries that provide sufficient range at a reasonable cost has gone on for a century. Electric power was a viable alternative when automobiles were first gaining popularity, eventually losing out to combustion engines in the 1920s. In recent decades, research efforts have gained greater urgency.
Like King Canute, who as ruler of England commanded the incoming tide to go out, the state of California decreed in 1990 that pollution-free electric cars must come into being. Battery-electric cars looked like a sensible solution for urban air-quality problems because pollutants would be produced where the electricity was generated, rather than where the car was driven.
Since the early 1990s the price of gasoline has doubled, and with it the motivation to seek alternatives. Battery technology has evolved considerably; hybrids have arrived, priced to reflect their need for two power plants instead of one and a battery that by itself is one-third of the car’s driveline cost. The plug-in hybrid — whose battery can be recharged from a wall socket as well as by an onboard combustion engine — has attracted a vocal following.
Before 1990, the principal battery choices were lead-acid, the familiar auto engine starting battery, and nickel-cadmium. The lead-acid battery is well-proved, but heavy considering the small amount of energy it can store. Nickel-cadmium batteries offer more miles of driving for a given weight and size, but are less attractive because a recycling system is not well-established. They are also at least four times as expensive.
The real force driving battery development has been portable electronics and cordless power tools, not vehicles. Both are high-volume applications. The workhorse here is the nickel-metal-hydride battery, which can store three times the energy of lead-acid cells in a package the same size. Nickel-metal-hydride is the most commonly used type of batteries in hybrids and electric-only vehicles because they are long-lived — Honda’s warranty for the Civic Hybrid battery runs 10 years/150,000 miles in some states — but are a great deal more expensive than a basic lead-acid battery. With a rapid recharge taking three hours, they were not the answer to California’s push for mainstream zero-emissions cars.
Another new battery type came along in 1991 — the lithium-ion battery. Its light weight — lithium is the third-lightest of elements — improved energy capacity for a given weight, and subsequent developments in electrode chemistry suggest that by 2010 it will be the winning technology for all applications. (It is already common in devices like cellphones and laptop computers.)
One problem has been durability, with early lithium-ion units tolerating only 750 cycles of discharge and recharge, or about two years of service, before deterioration of the terminals carrying power reduces charge capacity by 20 percent. A change from a terminal made of carbon to one made of lithium titanate spinel oxide holds the promise of increasing this to 9,000 cycles and 20 years’ use.
Many other battery chemistries exist — sodium-sulfur, nickel-zinc and nickel-iron — but the major contenders for use in electric vehicles remain the nickel-metal hydride and lithium-ion types.
Temperature control is an important consideration in the development of auto batteries; some cell or electrode types need to be warmer than others to function. And there are upper temperature limits — overheating and fires from lithium-ion batteries in laptop computers made headlines about a year ago.
All batteries slowly lose their charge to small internal currents, which generate heat just as a toaster does. If electrode deterioration increases this self-discharge current enough, catastrophic overheating can occur. Novel electrode chemistries or external control electronics promise to eliminate this hazard.
So far, lithium-ion batteries have gained capacity at the rate of 8 percent to 10 percent a year, doubling their ability to store energy over a decade. This and improved electrode chemistry have refreshed the appeal of the battery-electric car. Tesla, an electric-car startup that plans to start delivering its $98,000 Roadster this fall, has developed a power storage system of 6,831 lithium cells, each about the size of a AA battery, that it says will power the car 200 miles.
With the prospect of greater range, increased durability and their low cost to refuel, battery-electric vehicles start to look like just a bigger and practical power tool — one that may well make more sense than electric cars that use hydrogen fuel cells to produce power.
Would urban and suburban citizens buy lots of small electric vehicles at a price competitive with economy gasoline-powered cars? Have batteries matured enough to hit such a price point? Or will new emissions solutions make the small turbodiesel our first choice, as in Europe? It all comes down to price.
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