GM's Plug-In Hybrid

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These fuel savings would only be the beginning, many experts say. As battery technology improves, and if plug-in hybrids are successful, the high-volume production of batteries could lower their price. This would make it affordable to increase battery-pack size in successive generations and rely even less on gasoline. Eventually, the internal combustion engine could be made smaller and be used exclusively to recharge the batteries on long trips in a configuration called a "series" hybrid. "As time goes on, we may look at even more electrification of the vehicle, including solutions that use fuel cells," says Peter Savagian, director of hybrid power-train systems at GM. At some point, the internal combustion engine could be replaced entirely by large electric motors and battery packs, or by hydrogen fuel cells.

But plug-in hybrids still face challenges. Their environmental benefits will depend on the source of electricity used to charge the batteries. In areas powered primarily by coal, a plug-in might not be better for the environment than a good hybrid, says Nicholas Twork, a spokesman for Ford. Penney says that plug-ins will typically be better, however, since other areas in the United States rely on cleaner sources of electricity.

What's more, Penney says, "if millions of these things were produced 20 years from now, it would really enable renewable energies like wind to take off." Wind power requires ways of storing energy generated when demand for electricity is low, he says. The cost of the storage makes it hard for wind to compete with other sources of electricity. Millions of plug-in vehicles charging at night would essentially provide free storage.

But right now the biggest challenge to plug-in hybrids is the battery technology. First, batteries are expensive. Several companies sell kits for converting existing hybrids to plug-ins with larger battery packs, and these can add $10,000 to the price of a car (see "Plug-In Hybrids Are on the Way"). The plug-in hybrid application will also be much more demanding on batteries than ordinary hybrids are. In a car such as the Toyota Prius, the battery stays at about a 50 percent state of charge, and this varies only a few percent as it absorbs power from regenerative breaking and delivers it in short bursts to augment the gas engine. With plug-ins, batteries will be charged up to at least 95 percent and then deeply discharged during the first 20 miles of driving before settling into an ordinary hybrid mode. Such deep discharging typically decreases battery life. Automakers want batteries that can last 10 years or so even under these conditions so that consumers won't need to buy new battery packs a few years into owning a vehicle.

Automakers' newfound interest in plug-in hybrids is due in part to promising new battery technologies that could be less expensive and have longer lifetimes, and in part to their being safer than the lithium-ion batteries used in laptops today (see "Safer Lithium-Ion Batteries"). "We're encouraged by the improvements in the technology represented by hybrid batteries today," Savagian says. "And we're encouraged that there are many companies pursuing different formulas for them. But we want to give them cause to continue to press forward."