A company that supplies Chrysler with a camping trailer has come up with an intriguing invention to unshackle electric vehicles from a power outlet.

Electric Motors and Vehicles Company has a prototype of a trailer that has a 70 kilowatt-hour battery pack and small gasoline engine. The unit can be attached to an electric car while it drives and fully recharge the electric car's battery three times with six gallons of gasoline in the engine.

The trailer also has a motor to move the rear wheels so it takes no power from car to make it go – it just needs to be guided.

Wil Cashen, the founder and principal of the company, invented the trailer with the idea that it could serve as a range-extender for electric cars. The company already builds an off-road camper trailer for Mopar that is sold as an accessory, usually with Jeeps.

"We think we fit a wide variety of needed uses with this product, rather than pushing our ways into assumptions about how people might use it," he said.