At the Detroit Motor Show, Ford and General Motors unveiled two hybrid-powered cars that can be plugged into electric sockets for overnight charging. And they are not alone. Toyota, Honda and Nissan are also researching and developing plug-in hybrids.

Ford showed their technology in the Airstream, which is based on a crossover vehicle, and General Motors used another concept called the Volt, which uses the same platform as the Opel Astra.

Unlike battery-powered cars of the past, these concept vehicles have a good daily driving range because of what engineers call a "series hybrid" powertrain. An electric motor drives the wheels with no assistance from the normal petrol/diesel combustion engine. This contrasts with the Toyota Prius which uses battery and engine to drive the wheels.

The Airstream and Volt technology is different in that a small petrol engine in the Volt runs a generator which charges the batteries while a hydrogen fuel cell is used in the Airstream. This means that these cars do not have the limited range of the old electric cars.

General Motors is taking the challenge seriously. A chief engineer and vehicle line director have been appointed to the project, which is considered unusual and significant for a concept car. And the GM vice-chairman, Bob Lutz, has gone on record saying the Volt is being developed as a "production intent" vehicle.

On the road, the Volt's batteries are recharged by a turbo-charged 1-litre three-cylinder petrol engine connected to a generator. Under typical driving conditions the car can travel 64km before the petrol engine is called on to recharge the batteries.

Big developments are taking place in lithium-ion batteries and GM has given contracts to two firms to produce battery cells that are adequate for production cars.

Lithium-ion batteries store more energy than conventional lead-acid batteries, as used in today's family saloons, but generate more heat and sometimes catch fire. GM is speculating that by 2010 the batteries will be ready for mass production.

According to Lutz, "We are taking a calculated gamble on the batteries."

As fuel cells are still too expensive for volume production, Ford is also using a petrol engine to keep batteries charged, although the Airstream can cover 40km on battery and 490km with the fuelcell assistance.

Neither Airstream nor Volt have yet been approved for production.

The three big Japanese names have not yet declared they will develop production versions, although Toyota is working on a plug-in version of the Prius. Nissan has confirmed it is developing lithium-ion batteries but cautioned that it is years away from showing its plug-in technology. Honda is looking into plug-in technology but has no plans to display a concept soon.