Toyota Motor Corp unveiled a "plug-in" hybrid car based on its popular Prius model, saying it would test the fuel-saving vehicle on public roads -- a first for the industry.

But the world's biggest automaker said the car, called the Toyota Plug-in HV, was not fit for commercialisation since it uses low-energy nickel-metal hydride batteries instead of lithium-ion batteries believed to be a better fit for rechargeable plug-in cars.

Unlike earlier gasoline-electric hybrids, which run on a parallel system twinning battery power and a combustion engine, plug-in cars are designed to enable short trips powered entirely by the electric motor, using a battery that can be charged through an electric socket at home.

Many environmental advocates see them as the best available technology to reduce gasoline consumption and global-warming greenhouse gas emissions, but engineers say battery technology is still insufficient to store enough energy for long-distance travel.

"It's difficult to say when plug-in hybrids could be commercialised, since it would depend largely on advances in battery technology," said Executive Vice President Masatami Takimoto, in charge of Toyota's powertrain technology, told a news conference.

The Toyota Plug-in HV, which is due to be tested also in the United States and Europe, has a cruising range of just 13 km (8 miles) on one charge, even with its trunkful of batteries.

Detroit's General Motors Corp and Ford Motor Co are also working on plug-in hybrids, with cooperation from battery makers such as Germany's Continental AG and Korea's LG Chem. GM in January showed a concept version of the plug-in Chevrolet Volt that would be powered by a lithium-ion battery. It has set 2010 as a target for production.

Ford this month partnered with No 2 US electric utility Southern California Edison for real-world testing of a fleet of up to 20 rechargeable vehicles to be based on the Escape Hybrid SUV. Ford has said plug-ins could enter showrooms in five to 10 years.

Toyota, which launched the world's first mass-volume gasoline-electric hybrid car, the Prius, in 1997, said it would test eight prototypes of the plug-in hybrid to gather data on real-life driving over the next three years after gaining government approval on Wednesday.

Many automakers including Toyota, Nissan Motor Co. and Mitsubishi Motors Corp., are working with Japanese battery makers to develop next-generation lithium-ion batteries with improved capacity to store energy. Toyota has sold over half of its hybrid sales in the US market.

Jim Lentz, Toyota's US sales chief, said the automaker expected to sell 250,000 Toyota and Lexus-branded hybrids this year in the US market.-Reuters