Researchers at the University of Washington claim that “with a battery-free phone, you never have to worry about charging your phone again.”

Their findings, published in the June 2017 issue of the journal Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies, describes how the new phone powers itself in real time to make calls.

“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,” says Shyam Gollakota, a University of Washington associate professor who worked on the project.

Instead of using a battery, the phone collects energy from the radio signals that are transmitted back and forth among countless devices. Most radio signals, such as those given off by cell phones, radio towers, and wifi routers, lose much of their energy before reaching their target, so the battery-free phone isn’t draining anyone else’s signal when it takes in that energy and converts it to electricity.

As long as the phone is within 31 feet of one of those sources, it can collect enough power to make a Skype call, the researchers learned. By collecting ambient solar energy on top of the radio signals, that range can be as large as 50 feet.

The phone only requires 3.5 microwatts of power, so it can get by with a relatively weak signal — radio signals can contain three times that much energy. This is a far cry from a typical iPhone, which takes hours to charge using a 12-watt cable.

“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 is the cellphone,” said Joshua Smith, who led the research team.

The call quality is far from what you’d experience with an iPhone though, and the user of the phone needs to press a button in order to speak like with a walkie-talkie, but the researchers see their device as the first step toward a battery-free future. The demo video below shows how the phone can gather enough energy to transmit your voice and power your headphones during a call.