Battery-Free Cellphone Makes Calls by Harvesting Ambient Power

University of Washington

July, 2017
Battery-Free Cellphone Makes Calls by Harvesting Ambient Power
UW engineers have designed the first battery-free cellphone that can send and receive calls using only a few microwatts of power.Mark Stone/University of Washington

A team of electrical engineers and computer scientists at the University of Washington have invented a cellphone that is battery-free, signaling a major leap forward in discarding cords, chargers, and low-battery warnings. Instead, the phone harvests the few microwatts of power it requires from ambient radio signals and light.

“We’ve built what we believe is the first functioning cellphone that consumes almost zero power,” said Shayam Hollakota, an associate professor in the Paul G. Allen School of Computer Science & Engineering. “To achieve the really, really low power consumption that you need to run phone by harvesting energy from the environment, we had to fundamentally rethink how these devices are designed.”

The cross-disciplinary team eliminated a power-hungry step in most modern cellular transmissions – conversion of analog signals that convey sound into digital data. As that process consumes so much energy, it has been the barrier to ambient powered devices. To solve that, they took advantage of tiny vibrations in a phone’s microphone or speaker that occur when a person is talking or listening on a call. An antenna on those components converts that motion into an analog radio signal emitted by a cellular base station – encoding speech patterns in reflected radio signals.

“The cellphone is the device we depend on most today. So if there were one device you’d want to be able to use without batteries, it’s the cellphone,” said electrical engineering professor Joshua Smith. “The proof of concept we’ve developed is exciting today, and we think it could impact everyday devices in the future.”

The prototype battery-free phone does still require 3.5 microwatts of power, which the researchers demonstrated could be harvested from either ambient radio signals or with a tiny solar cell – the size of a grain of rice.

While going battery-free on many wearable or portable technologies can be accomplished by use of temperature sensors or accelerometers, they are able to conserve power by taking a reading and then resting while harvesting information before going to the next task. However, for a phone, the device has to operate continuously for as long as a conversation lasts.

“You can’t say hello and wait a minute for the phone to go to sleep and harvest enough power to keep transmitting,” said co-author Bryce Kellogg, a UW electrical engineering doctoral student. “That’s been the biggest challenge – the amount of power you can actually gather from ambient radio or light is on the order of 1 or 10 microwatts, so real-time phone operations have been really hard to achieve without developing an entirely new approach to transmitting and receiving speech.”

To learn more about this project and their additional efforts to stream video of this phone, click here.

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