The First Battery-Free Cellphone

Alasdair Allan
3 min readJul 6, 2017

The University of Washington has a long history with energy harvesting and battery-free computing. One of their more notable projects was their work on ambient backscatter technology. There, they used existing TV and cellular transmissions — ambient RF energy that’s already in the air around us—as a source of power, but also as the communication medium.

This time, they’ve gone ahead and built a battery-free cellphone with the phone harvesting the few microwatts it needs from either ambient radio signals, or light.

The battery-free cellphone prototype. (📷: Mark Stone/University of Washington)

One of the things holding back both the widespread deployment of Internet of Things smart devices, and mobile computing in general, is the limitations of battery technology. Lagging behind the developments in processing, storage, and displays, battery technology isn’t keeping up with the power needs of modern computing.

Reducing the power requirements for devices rather than adding more, and bigger, batteries as we’ve been doing up till now will mean that IoT devices are smaller and cheaper. Which means they’ll be more widely deployed, and again, cheaper still.

While battery-free sensors—like the Columbia University’s self-powered camera—do exist, a lot of these sensors reliant on ambient energy sources conserve power by intermittent operation. They take a reading and then sleep for a few minutes while harvesting enough energy to perform the next measurement. A cellphone needs to operate continuously for as long as a call lasts, and that isn’t an option.

A look at the battery-free cellphone. (📹: University of Washington)

Based around an MSP430, and using off-the-shelf components, the research team have built a prototype phone that can perform the basic phone functions —allowing a user to dial a number, and then transmit and receive both speech and data to a specialised basestation. The basestation then uses Skype to dial out, and receive incoming calls, on the phone.

…the baŠttery-free cellphone communicates exclusively with [a] basestation which is responsible for bridging the connection between the phone and cellular network. ŒThe phone uses analog backscattŠer to directly transmit speech from a passive microphone and amplitude modulation to receive analog speech from the basestation. ŒThe phone uses existing digital back scatterŠer techniques to communicate state information and co-ordinate with the basestation. BackscatterŠ Štransmitters are 3–4 orders of magnitude lower power compared to radios and minimise the power consumption of the phone.

The phone can send and receive calls using only a few microwatts of power. (📷: Mark Stone/University of Washington)

The team’s future plans are to improve the phone’s operating range and to encrypt conversations to make them secure. However they’re also working to stream video, and add a display to the phone using low-power E-ink screens.

Interested in learning more about the battery free cellphone? You can get more information from the team’s paper. If you’re also interested in their previous work on ambient backscatter technology, you can find more details on that from the original paper.

[h/t: University of Washington]