Everyone has felt the pain of a device running out of power just when you need it the most.

That pain point is fueling research on dozens of inventive charging technologies, but one group of researchers at the University of Washington is taking a different approach: get rid of the battery altogether.

The group unveiled its battery-free cellphone, which uses energy from ambient light and radio waves, in a recent paper. Although the current model is still a prototype, the group has passed the biggest hurdle in making the technology a reality: converting analog sound signals into digital signals that a phone can process.

“We’ve built what we believe is the first functioning cellphone that consumes almost zero power,” the paper’s co-author, Shyam Gollakota, said in a UW release. Gollakota is an associate professor in the UW’s Paul G. Allen School of Computer Science & Engineering and a recent Geek of the Week on GeekWire.

“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,” he said.

Check out the video below for a demonstration of the device.

The group built the phone using off-the-shelf computing components and a custom printed circuit board. It takes about 3.5 microwatts to run, which the phone can get either from radio waves sent out by a custom-built base station or from a tiny solar cell, about the size of a grain of rice, attached to the phone.

Here’s how it works: when someone speaks into the phone, an antenna picks up vibrations in the device’s microphone which are then encoded in radio signals. The phone receives communication via encoded radio signals and translates it into vibrations in the phone’s speaker.

Both operations take very little power and let researchers make phone calls through the base station, which could be integrated into existing cell service infrastructure to allow the phone to be used more widely.

On the current prototype, a user must press a button to move between sending and receiving modes, and the phone needs to be within about 30 feet of a radio base station to operate. The group plans to improve those aspects of the device going forward.

The researchers previously developed battery-free wireless communicators designed for internet of things devices.