Toyota yesterday unveiled a "plug-in" hybrid car based on its popular Prius model on Wednesday, 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 commercialization 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 not up to storing enough energy for long-distance travel.

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

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

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.