PHOTO: General Motors

Chevrolet's Volt is the first series hybrid concept car shown by a major manufacturer. In a series hybrid, the engine's only job is to crank a generator; electric power does all the rest of the work.

In late November General Motors announced plans to release a vehicle that will be able to go long distances in electric-only mode. It thus became the first U.S. company to commit to producing a so-called plug-in hybrid design—one that has batteries so capacious that they can be recharged not only by the engine but also from wall current in the garage. It represents the next way station along the path to an all-electric vehicle.

Troy Clarke, president of GM North America, told IEEE Spectrum that a plug-in version of the Saturn Vue Green Line sport-utility vehicle could hit dealer lots 24 months after the launch, in 2009, of a standard hybrid version using GM's "two-mode hybrid" transmission. He would not, however, commit to a specific date or even a year.

Tellingly, GM has not yet announced where it will get the lithium-ion batteries that any plug-in requires. Only such batteries—the kind used in laptops—pack enough energy to sustain electric-only mode for 32 kilometers (20 miles), the range generally regarded as necessary. In a statement released on 4 January, in the runup to the Detroit Motor Show, the company did say that it had agreed to support the battery technology programs of two joint ventures, and that it would also assess the technologies of other, unnamed companies.

Beyond plug-ins: the Volt

Although plug-in hybrids involve larger batteries, their fundamental design hardly varies from that of other, mechanical-drive cars. More radical is the “series hybrid electric” car, which powers the wheels with electric motors and uses the onboard combustion engine only to run a backup generator that recharges the batteries as needed.

The Chevrolet Volt, unveiled to the press on 7 January at Detroit’s North American International Auto Show, is the first-ever series hybrid concept car shown by a major manufacturer. For an animated tour of its innards, click here. Its 1.0-liter, 3-cylinder turbocharged engine runs an onboard 53-kilowatt generator that recharges a 16-kilowatthour lithium-ion battery made of 80 four-volt cells. The battery pack’s volume is 100 L, one-third as much as the lead-acid batteries in GM’s 1990s-issue electric car, the EV1. GM’s targeted maximum weight for the pack is 180 kilograms (400 pounds). The company also wants the battery to last at least 10 years, through 4,000 full-discharge cycles.

The battery pack would charge in less than 6.5 hours, power a 120-kW electric motor delivering 320 newton-meters of peak torque, and go 64 km (40 miles) in all-electric mode on battery charge alone. The 12-gallon gasoline tank would add an additional 965 km (600 miles) to that range.

“We don’t have a battery pack yet,” said Tony Posawatz, the vehicle line director. He confirmed that the vehicle shown in Detroit doesn’t yet run.

Lithium ion: light and cheap

Everything thus depends on the pace of development of lithium-ion batteries. Right now they’re the only candidate for the job, because they store more than twice as much energy (110 to 130 watt hours per kilogram) as the next-best technology, the nickel-metal-hydride (NiMH) batteries in today’s gas-electric hybrids. The reason: lithium is the lightest solid element, so it’s easily portable. What’s more, it’s cheap.

To make lithium-ion batteries practical for mass-produced electric-drive vehicles, new technologies must increase the energy the batteries store and the speed with which they can discharge it. They must also lengthen cycle life to 15 years or 241 000 km (150 000 miles)—the average life of a vehicle. Finally, they must keep the cost as low as possible.

The technology has advanced quickly, says Mark Duvall, manager of technology development for electric transportation at the Electric Power Research Institute, in Palo Alto, Calif. He’s “impressed and bullish” on the prospects for new lithium variants, some of which EPRI has tested to ascertain their cycle lives.

The first production car to use lithium-ion batteries was the Toyota Vitz CVT 4, a small car sold only in Japan. It used a four-cell, 12 ampere-hour lithium-ion battery pack to power its electric accessories and restart the engine after idle stop. More recently, Tesla Motors, in San Carlos, Calif., has offered the Tesla Roadster, an all-electric sports car that uses 6831 lithium-ion cells, each roughly the size of a double-A battery. They give the car up to 400 km (250 miles) of range, as well as the breathtaking acceleration of 0 to 100 kilometers per hour (0 to 60 miles per hour) in less than 4 seconds.

Why use so many little cells? First, because they’re readily available, and second, because current lithium technology is susceptible to thermal runaway—a problem underlined recently by flaming laptops—and larger cells mean greater risk. The Tesla’s 410-kg (900-pound) battery pack is stuffed not only with cells but also with sensors and control logic designed to detect and isolate any misbehaving cell.