Today at the North American International Automotive Show, in Detroit, Toyota announced that later this year, it will release a version of the Prius hybrid car whose battery can be recharged from an ordinary power outlet. By moving up the delivery data of the plug-in vehicle–originally scheduled for 2010–Toyota has slipped ahead of GM, whose Chevy Volt plug-in is promised for late 2010.

Toyota’s fidelity to hybrid technology marks a sharp contrast with rivals such as Renault and Mitsubishi, which are planning to leapfrog the hybrid in favor of fully battery-powered electric vehicles (EVs). At the auto show, several U.S. automakers appear to be leaning in the same direction, with Ford Motor, in particular, vowing to release an EV commercial van next year and an EV commuter car in 2011.

Even Toyota is hedging its bets, presenting a battery-powered EV based on its four-seat iQ and promising to begin selling a similar EV commuter car in the United States by 2012. But Toyota explicitly ruled out abandoning hybrid technology anytime soon, issuing a definitive statement on the eve of the Detroit show calling hybrids its “long-term core powertrain technology.”

The 2010 Prius available to consumers will still come equipped with a nickel-metal-hydride (NiMH) battery pack and no plug, but Toyota says that it is “plug-in ready”–designed and engineered to accept a lighter and more energy-dense lithium-ion battery pack that can be charged from the grid. Toyota will also produce 500 lithium-powered plug-in Priuses for its commercial and government leasing customers starting later this year. Toyota-Panasonic joint venture Panasonic EV Energy will supply the lithium batteries.

The fact that the plug-in battery pack can be swapped in for an ordinary hybrid battery suggests that it will be relatively small, and that the plug-in Priuses will have a smaller electric-only range than the Volt and the Chinese-built BYD F3DM. The plug-in vehicles that Toyota has been testing in Japan, France, California, and the United Kingdom are Priuses equipped with a second NiMH battery pack that gives them less than 10 miles of electric-only range.

Limiting the battery size is a conscious decision. Recent studies suggest that a limited amount of electric-only range may be optimal for plug-ins, especially in the United States, where half of electricity generation is coal fired. A working paper by Duke University researchers, released in November, predicts that plug-in hybrids storing enough electricity to travel 40 miles on a charge–like the Chevy Volt–will offer few if any greenhouse-gas reductions relative to conventional hybrids. Such long-range plug-ins will likely also cost more per mile, thanks to the high price of the lithium-ion batteries required to store the electricity.

In contrast, research to be delivered today by Carnegie Mellon University mechanical engineer and design expert Jeremy Michalek at the National Academy of Sciences’ Transportation Research Board meeting finds that plug-ins with just 20 miles of electric range are likely to best conventional hybrids in both cost and carbon footprint–assuming they are charged frequently.

That’s because most trips are short. Vehicles driving less than 30 miles a day account for about 60 percent of annual U.S. passenger-vehicle miles, according to the U.S. Department of Transportation. Meanwhile, batteries are heavy. Michalek says that plug-in batteries, plus the structural material to support them, could substantially increase the weight of a car. “If you’re carrying around batteries that you’re not using, you’re spending more money and actually hurting the planet,” says Michalek.

Etienne Plas, a European spokesman for Toyota, says that three plug-in Priuses test-driven by employees of power-company Electricité de France since November 2007 are demonstrating a significant improvement in fuel efficiency. “In France, the early testing indicates that for trips up to 25 kilometers in duration, we see fuel efficiency up to 60 percent better than a regular Prius,” says Plas. “That’s quite significant.”

Plas says that Toyota is still studying how frequently test drivers plug their cars in–a variable that, as Michalek notes, will have a large impact on the plug-ins’ real fuel efficiency. But even consumers who never plug in are likely to see some benefit from the low-range Prius, at least when they’re driving in town. The Prius already captures energy from braking to recharge the battery. According to Plas, that energy can actually max out a small battery in stop-and-go driving. “With the current Prius, if you are in really heavy city traffic and you keep on braking, you will not be able to capture all the energy,” he says.

Such economic and environmental details could make the difference for automakers in the months and years ahead, as they seek to woo consumers back to showrooms. Auto sales in the United States in November were 37 percent lower than in 2007, according to industry tracking firm Autodata. Relatively pricey hybrids were hit even harder, slumping 53 percent relative to November 2007.

This spring, Toyota plans to shut down all of its Japanese production for 11 days to reduce the resulting supply glut; most automakers have taken similar steps. But Toyota remains convinced that high gas prices will be back to drive the continued electrification of the industry as a whole. As Irv Miller, vice president for environmental and public affairs at Toyota Motor Sales USA, said in Detroit this past weekend, “Last summer’s four-dollar-a-gallon gasoline was no anomaly. It was a brief glimpse of our future.”