If you've ever been caught out with a dead phone battery in the middle of nowhere, take heart. A team of researchers from the University of Washington (UW) have developed a phone that requires no batteries. Still in the prototype phase, the basic phone harvests the minimal power it needs from ambient radio signals, yet can communicate with a base station to make voice calls.

Smartphones may be modern miracles, but their battery life, or lack thereof, can be aggravating, with many users unable to make it through a single day without having to seek out a charger and wall outlet.

This is because phones are power hungry with many always-on circuits. One of the biggest power draws is converting the analog sound signals of a voice into digital data. Though phone prototypes have been produced that harvest energy from ambient power sources to extend battery life, it hasn't been possible to make one that dispenses with the battery altogether.

According to the UW team, their new battery-less phone consumes almost zero power. Built on a printed circuit board using off-the-shelf components, it looks deceptively simple, but it's capable of very basic functions – send, receive, make and accept calls, and place callers on hold.

When a person talks or listens on a phone, tiny vibrations occur in the microphone or speaker, which for the prototype is a pair of earphones. Instead of transmitting its own radio signals, it piggybacks on the back of the radio signal emitted from a bespoke cellular base station.

To transmit speech, an antenna connected to the microphone encodes the user's speech patterns in the radio signal reflected from the antenna, while receiving speech involves an antenna converting speech patterns encoded in the radio signals into sound vibrations that are relayed to the phone's speaker. Like a walkie talkie, the user must press a button to switch between listening and transmitting modes.

While battery-less, the system still needs about 3.5 microwatts to function. It harvests this from the ambient radio signals of the base station at a distance of up to 31 ft (9 m). Using a solar cell about the size of a grain of rice to gather ambient light increases the range to 50 ft (15 m).

Though the current system is a laboratory lash up, the researchers believe that the technology could be installed in a standard cellular network or Wi-Fi router to make calls battery-free. However, there are still a number of technical hurdles to overcome. For example, other ambient energy technologies harvest heat or motion, which allows them to soak up power while they are in sleep mode before performing a task. The UW phone, on the other hand, has no ability to sleep and harvest energy during a conversation.

"You can't say hello and wait for a minute for the phone to go to sleep and harvest enough power to keep transmitting," says Bryce Kellogg, a UW electrical engineering doctoral student. "That's been the biggest challenge – the amount of power you can actually gather from ambient radio or light is on the order of one or 10 microwatts. So real-time phone operations have been really hard to achieve without developing an entirely new approach to transmitting and receiving speech."

In addition to the power harvesting problems, the team is also working on how to encrypt messages and stream videos, and adding a low-power E-ink screen for a display.

The video below shows the battery-free phone in action.

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