Inside the battery-free cell phone of the future

Batteries: You can't live with them, you can't live without them, and I recite that cliche because I know you're nodding along with me right now. Smartphone battery packs have become bigger so that they last longer, and a majority have been infused with fast charging technology. But a dead battery remains a major pain point, especially since we've become so reliant on mobile devices.

But what if you didn't have to worry about the battery at all? Or ensuring it's charged every night? What if you didn't have to fret about running out of juice in the middle of the day? University of Washington researchers are working toward that reality. They've discovered a way to harvest power for the smartphone without cables and physical battery packs. Instead, the phone relies solely on ambient radio signals and light. It's exactly what it sounds like: a battery-free smartphone.

Zero power required

Photo credit: Mark Stone/University of Washington

Photo credit: Mark Stone/University of Washington

Researcher Shyam Gollakota, an associate professor at the University of Washington, called the battery-free cell phone the "first functioning cell phone that consumes almost zero power." It works by taking advantage of the tiny vibrations in a phone's microphone and speaker and converting those motions into an analog radio signal that can communicate through a cellular base station. "This process essentially encodes speech patterns in reflected radio signals in a way that uses almost no power," explains the news release. "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 are picked up by the phone's speaker."

The first functioning cell phone that consumes almost zero power.

The device currently requires you press a button to switch between transmitting and listening modes, so it's not exactly a touch-and-go experience like a regular smartphone. But in the future, the technology could be integrated into standard cellular network infrastructure and Wi-Fi routers to offer battery-free cell phone coverage.

I talked to Vamsi Talla, a research associate on the battery-free project, about the battery-free cell phone and what it takes to power such a device. "Our group had been working on battery-free devices and low power communication for the last five years," said Talla. "This projects builds on some techniques that we have developed. For example, the battery-free phone uses a zero power analog backscatter microphone, which I developed in 2012. Then, we combined that with an analog headphone system and ambient backscatter communication."

Talla and the research group, which also consists of two graduate students and two professors, initially started the project in early 2016. "I built two different hardware versions of the phone prototype," he said. The initial one was powered by ambient RF signals transmitted through cell towers. The second added "a tiny photodiode (solar cell) to harvest energy from ambient light in a typical office or home setting." In situations where it's RF-barren — say, you're in the forest or the middle of nowhere — the phone will harvest the light around you for power.

Photo credit: University of Washington, Battery-Free Cellphone

The battery-free phone prototype is comprised of a number of components you used to be able to buy at a Radio Shack: antennas, a power harvester to capture energy from ambient RF signals and solar cells, a microphone that listens for reflections and backscatter to record and transmit speech, a receiver, a headphone jack, a digital communication system, a small micro-controller, and a few LEDs and capacitive touch buttons for help with operating the device. Don't expect to be playing games on the device anytime soon, however, as there's not enough energy generated to power that sort of thing. "Unless we redesign that component, it might be hard to power it only with RF waves," added Talla.

The battery-free phone prototype is comprised of a number of components you used to be able to buy at a Radio Shack.

Current phones consume up to ten-thousand times more power than what's available from ambient light and RF signals. The research team had to reduce the power required to transmit and receive speech to a few microwatts. It was no small feat, explained Talla. "The amount of power that can be harvested from RF signals depends on the distance between the phone and the cell tower. Typically, one to 100 microwatts can be harvested from ambient RF signals. We redesigned the battery-free phone's architecture to reduce the power consumption by about 10,000 times so that it can be continuously powered from the tiny amount of power that is available."

The research team's next objectives are to improve the battery-free phone's operating range and ensure that conversations are encrypted.

If you'd like to know more about the battery-free unicorn — er, I mean, mobile phone, you can peruse the full research paper for yourself, which was published in early July. You can also read more at the Battery Free Phone project's official page.

Florence Ion

Florence Ion was formerly an editor and columnist at Android Central. She writes about Android-powered devices of all types and explores their usefulness in her everyday life. You can follow her on Twitter or watch her Tuesday nights on All About Android.