"Hello, hello. I am calling from a battery-free phone." Vamsi Talla's words in a cluttered lab at the University of Washington in Seattle are barely audible through pops and static. But the fact they can be heard at all, on a nearby Android smartphone, is revolutionary, because Talla's own cell phone has no battery at all. It draws what little power it needs from thin air.

The prototype cell phone is the culmination of a years-long quest by Talla, a research associate at the lab of Joshua Smith, who researches computer science and electrical engineering at UW. "If you had to pick one device to make battery-free, what would you pick," asks Smith. "A cell phone is one of the most useful objects there is. Now imagine if your battery ran out and you could still send texts and make calls."

Realizing that vision required rethinking almost everything about how cell phones function today. In order to operate without a battery, the phone would have to rely only on energy that it could harvest from its surroundings.

Ambient light can be turned into a trickle of electricity with solar panels or photodiodes. Radio-frequency TV and Wi-Fi broadcasts can be converted into energy using an antenna. A hybrid system using both technologies might generate a few tens of microwatts. The problem is that a traditional cell phone uses tens of thousands of times more power, around 800 milliwatts, when making a call.

Power Up

The first thing the team tackled was communication. Smith's lab developed a technique called backscatter that allows a device to communicate by reflecting incoming radio waves, a bit like an injured hiker sending an SOS using the sun and a mirror. Smith has already spun out a start-up called Jeeva Wireless to commercialize what he calls "passive Wi-Fi"—digital backscatter technology for ultra-low power Wi-Fi gadgets. However, even passive Wi-Fi proved too power-hungry for the cell phone project.

"Converting analog human speech to digital signals consumes a lot of power," says Talla. "If you can communicate using analog technology, you're actually more power efficient." So although the cell phone uses digital signals to dial numbers, the backscatter process for voice calls is purely analog.

While developing analog backscatter, Smith realized that he was essentially re-inventing a spy technology used during the Cold War. In 1945, the Soviets presented the American Ambassador in Moscow with a carving of the Great Seal of United States. Inside was hidden an audio bug that only activated when illuminated by the correct frequency of radio waves, using the energy of the waves themselves to operate.

"My dad was a spy in the Cold War, so I heard stories about the Great Seal bug when I was a kid," says Smith. "I wondered if analog backscatter could be software-controlled and turned from a curiosity for spooks into a technology that everyone could use."

Like the bug, some key components of Talla's phone are housed remotely to save power. A nearby basestation has circuitry for converting and connecting to the digital cellular network, currently via Skype. The prototype basestation uses an unlicensed frequency, limited to low-power transmissions. Because the phone relies on those signals for its energy harvesting, it has a range of just 15 meters from the basestation.

To develop the phone commercially, that circuitry could be built into a Wi-Fi router at home or, more likely, a traditional phone tower. "Real cell towers have a hundred times as much power, and would increase the range to perhaps a kilometer," says Talla.

Watts Happening

There is still a long way to go before that happens. The phone has a basic touch-sensitive number pad and its only display is a tiny red LED that glows briefly when a key is pressed. A large touchscreen would require around 400 milliwatts—over one hundred thousand times as much as power as Talla's phone currently needs.

Most importantly, voice calls are still awkward. You have to press a button, walky-talky style, to switch between listening and talking, and sustaining a conversation through clouds of static is near impossible.

Talla promises better call quality and an E-Ink display for text messages on the next generation device, possibly along with a camera to snap selfies. Smith says that even as the prototype stands, built from off-the-shelf components, it is much cheaper than a normal phone. Built at scale, it would be cheaper still. And better yet, you would never have to worry about leaving your charger at home ever again.