Even the most intelligent robot is useless without power.

While advanced robots like Honda's Asimo regularly astonish audiences with their dexterity, the power-hungry humanoid needs a change of battery after only 40 minutes.

For future robots to perform useful tasks, such as undertaking construction work or exploring disaster zones, they will need smarter ways to keep their batteries topped up when they are out of reach of a mains-powered battery charger, or an obliging human.

"No robot can be truly autonomous if it cannot revive itself after its battery has expired," says Trung Dung Ngo, a robotics researcher at Aalborg University in Denmark. Too much research, he says, is directed at giving robots behavioural autonomy, instead of energy autonomy. "They must have a means to self-power," he says.

So Ngo and his colleague Henrik Schioler have come up with a plan whereby a sizeable "mother ship" robot will continually replace ...