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The newly developed battery-free cellphone can send and receive calls using only a few microwatts of power.

Credit: Mark Stone/University of Washington

Imagine being out and about, only to realize that your phone's battery life is running dangerously low and there's nowhere nearby to charge it. Now imagine how liberating it could feel to not have to worry about that. A new cellphone prototype could one day provide such relief because it doesn't need a battery at all, according to a new study.

The phone, a voice call-only device, is by no means the sexiest cell on the block — the calls crackle and the phone only works within a stone's throw of a computer that serves as a sort of cell tower. But how does the device work without a battery?

The cellphone requires such little power — only a few microwatts rather than the 100 microwatts a smartphone uses for voice calls —  that the power it does need can be collected from the environment, according to the researchers. A tiny photodiode, smaller than an adult's pinky nail, collects ambient light while a radio frequency harvester makes it possible to use energy sent out wirelessly from a homemade cell tower, called a base station.

To make even such a simple-sounding phone — one that doesn't draw on a battery — required the phone's developers, a team of researchers from the University of Washington, to overcome a hurdle inherent in other battery-free devices.

The trick others have used to enable devices to work without a battery is to alternate periods of activity with periods of energy collection. That is, the devices would switch off periodically, which, while practical enough for a camera or a temperature sensor, would be maddening for a phone.

To keep their phone working continuously, the researchers chose perhaps a counterintuitive approach: to go analog. The battery-free cellphoneincorporates a technology called analog backscatter, a way to absorb or reflect a signal that requires less power than generating a signal, in the same way using a mirror to reflect the light from a flashlight takes less power than generating the light in the first place.

"By doing the signals in an analog way, we actually got the power consumption so low that you never have to turn off your phone," Vamsi Talla, one of the phone's designers and a computer science and engineering research associate at the University of Washington, told Live Science. The power-heavy work of converting analog signals into digital ones is then outsourced to the in-lab base station.

"It's the first type of their system in the world that demonstrates that you are actually able to make a phone call with just a microwatt power consumption," Pengyu Zhang, a postdoctoral researcher in electrical engineering at Stanford University who was not involved in the study, told Live Science. "That's amazing."

Zhang does see one major obstacle in the way of battery-free phones becoming commonplace, though.

"If you look at a real cellphone base station, it involves two links: uplink from the cellphone to the base station and downlink from the base station to the cellphone," he said. "You have to enable the communication in both directions. However, if you implement the communication — the downlink, where the base station talks to the cellphone — the power consumption of the cellphone is actually very, very high. And I’m not sure how can you enable such capabilities with this design."

Talla acknowledged there's still a long way to go, particularly when it comes to integrating the technology into cell towers, but he said he's hopeful 5G networks — the next level of telecommunications standards — might help make commercial battery-free cellphones a reality. He imagines that someday people might own both a battery-powered smartphone and a battery-free phone.

"Let's say your phone is low on battery or the battery has died, then you can use this phone, at least, to make an emergency 911 phone call," Talla told Live Science. "That could be a lifesaver in a lot of scenarios."

The new study was published online June 30 in the journal Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies.

Original article on Live Science.