CHICAGO — Plug-in hybrids are popping up all over the auto-show circuit.

Saturn says it hopes to sell a Vue plug-in with a 10-mile electric-only range by 2010, and Toyota says it will have a few hundred plug-ins in commercial test fleets by then.

Others are trying to get in the game. AFS Trinity Power Corp. of Bellevue, Wash., has equipped a 2007 Vue with a prototype plug-in system it says can go 40 miles on lithium-ion batteries before a 4-cylinder engine kicks in for gas/electric operation.

Estimated cost is $8,700 for the Extreme Hybrid system — based on an automaker producing 10,000 or more cars per year (production costs go down as volume goes up).

The 40-mile electric range equals the target set by General Motors for the Chevrolet Volt plug-in it hopes to produce starting in 2010 if lithium-ion batteries are ready. Plug-ins can recharge their batteries with household current.

The difference with AFS's system, said chief technology officer Donald Bender, is, "We're doing it with batteries available now. We're not waiting for the technology to develop."

AFS uses ultra-capacitors and proprietary electronics to boost battery power and prevent overheating, a key concern with lithium-ion. Bender said the system can be adapted to "just about any" sport utility vehicle. AFS plans a similar system for passenger cars.

"My next dream is that one of the (automakers) will say they want to put this system on their car," AFS Chairman and Chief Executive Edward Furia said. "We could give them exclusive use for two years, and then we could license it to others."

Furia said consumers could recover the $8,700 cost in fuel savings in 3 1/2 years with gas at $2.85 and in less time at higher prices.

Still David Champion, of Consumer Reports magazine, is skeptical of the hype around battery technology. "A breakthrough in battery technology has been two years away for 20 years."

That could be why Toyota is testing a handful of Priuses with a plug-in system that can go seven miles on its nickel-metal-hydride batteries before the gas/electric system kicks in.

"Cost is a big part of our research," said Jaycie Chitwood, a planner for Toyota. "We want to find out how much (electric-only) range people want and how much they want to pay for it. The batteries aren't free."

The current Prius does not have to be plugged in to recharge. It operates in an electric-only mode at lower speeds, with the gas engine the primary power source for most driving. The electric motor provides a boost for passing, and the engine and regenerative braking recharge the batteries.

Still the Toyota plans to have lithium-ion batteries in a plug-in test fleet in 2010. Lithium-ions are smaller, lighter and more powerful — and, right now, much more expensive.