Researchers at the University of Washington (UW) have invented a phone that harvests power it requires from either ambient radio signals or light and requires no batteries.

Detailed in a paper published in the Proceedings of the Association for Computing Machinery on Interactive, Mobile, Wearable and Ubiquitous Technologies, the battery-free cellphone marks a major leap forward in moving beyond chargers, cords and dying phones.

The researchers eliminated a power-hungry step in cellular transmissions - converting analog signals that convey sound into digital data that a phone can understand. This step typically consumes so much energy that it's been impossible to design a phone that can rely on ambient power sources.

Instead, the new technology takes advantage of tiny vibrations in a phone's microphone or speaker that occur when a person is talking into a phone or listening to a call.

An antenna connected to those components converts that motion into changes in standard analog radio signal emitted by a cellular base station. The process essentially encodes speech patterns in reflected radio signals in a way that uses almost no power.

To transmit speech, the phone uses vibrations from the device's microphone to encode speech patterns in the reflected signals.

To receive speech, it converts encoded radio signals into sound vibrations that that are picked up by the phone's speaker.

The UW team demonstrated that the prototype can perform basic phone functions, transmitting speech and data and receiving user input via buttons.

"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.

"To achieve the really, really low power consumption that you need to run a phone by harvesting energy from the environment, we had to fundamentally rethink how these devices are designed."

However, the phone does require a small amount of energy to perform some operations. The prototype has a power budget of 3.5 microwatts.

The researchers demonstrated how to harvest this small amount of energy from two different sources. The phone prototype can operate on power gathered from ambient radio signals transmitted by a base station up to 9.45 meters away.

And using power harvested from ambient light with a tiny solar cell, roughly the size of a grain of rice, the device was able to communicate with a base station that was 15.24 meters away.

The team plans to focus on improving the battery-free phone's operating range and encrypting conversations to make them secure.