Engineers have created a battery-free phone that can make calls is powered only by the ambient energy around it.

If that sounds absolutely mad, then that’s because it is.

The team from the University of Washington were able to develop a primitive phone that can harvest the microwatts of power it needs from radio signals around it or light.

What’s perhaps even more impressive is the team were able to make Skype calls from the device as well.

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“We’ve built what we believe is the first functioning cellphone that consumes almost zero power,” said co-author Shyam Gollakota, an associate professor in the Paul G. Allen School of Computer Science & Engineering at the UW.

So how can a phone function without needing a battery? The team had to radically rethink how a phone operates.

The phone is able to harvest energy from either radio signals produced by a based station up to 50 feet away. Or alternatively it can create electricity from a solar cell that’s no bigger than a grain of rice.

One of the most power hungry functions of a phone is when it converts the analogue signal that conveys sound into a digital one that a modern smartphone can understand.

To bypass that step the team took advantage of the tiny vibrations that occur in a phone’s microphone or speaker when a person is either listening or talking.

They can essentially convert that motion into a signal that can be sent or received using almost no power.

One disadvantage is that the user needs to press a button that switches the phone between talking or receiving, much like a two-way radio.

To make their prototype work the team effectively created their own base station, but have confirmed that it could be fitted into modern day cellular base stations as well.

“You could imagine in the future that all cell towers or Wi-Fi routers could come with our base station technology embedded in it,” said co-author Vamsi Talla. “And if every house has a Wi-Fi router in it, you could get battery-free cellphone coverage everywhere.”

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