Unlike traditional mobile phones, no batteries are necessary. The device, made from commercial, off-the-shelf components, harvests the few microwatts of power it requires from either ambient radio signals or light.

According to Shyam Gollakota, an associate professor of computer science and engineering, this breakthrough is “a major leap forward in moving beyond chargers, cords and dying phones.

“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,” Gollakota points out. He and his “U-Dub” colleagues eliminated a power-hungry step in most modern cellular transmissions—converting analog signals that convey sound into digital data that a phone can understand.

“This process consumes so much energy that it’s been impossible to design a phone that can rely on ambient power sources,” claims Gollakota.

Instead, the battery-free cellphone takes advantage of tiny vibrations in a phone’s microphone or speaker that occur when a person is talking or listening to a call.

“An antenna connected to those components converts that motion into changes in standard analog radio signals emitted by a cellular base station,” says Gollakota. “This process essentially encodes speech patterns in reflected radio signals in a way that uses almost no power.”

To transmit speech, the phone uses vibrations from the device’s microphone to encode speech patterns in the reflected signals. To receive speech, it converts encoded radio signals into sound vibrations that are picked up by the phone’s speaker.

“[We] designed a custom base station to transmit and receive the radio signals,” explains Gollakota. “But, that technology conceivably could be integrated into standard cellular network infrastructure or Wi-Fi routers now commonly used to make calls.”

The prototype phone can operate on power gathered from ambient radio signals transmitted by a base station up to 31 feet away. However, using power harvested from ambient light with a tiny solar cell the size of a grain of rice, the device was able to communicate with a base station that was 50 feet away.

“Many other battery-free technologies that rely on ambient energy sources, such as temperature sensors or accelerometers, conserve power with intermittent operations,” notes Gollakota. “They take a reading and then ‘sleep’ for a minute or two while they harvest enough energy to perform the next task. By contrast, a phone call requires the device to operate continuously for as long as the conversation lasts.

“You can’t say ‘hello’ and wait for a minute for the phone to go to sleep and harvest enough power to keep transmitting,” adds Gollakota. “That’s been the biggest challenge.

“The amount of power you can actually gather from ambient radio or light is on the order of 1 or 10 microwatts,” says Gollakota. “So, real-time phone operations have been [difficult] to achieve without developing an entirely new approach to transmitting and receiving speech.”