Have you ever thought of a mobile device that runs without battery? Researchers say it's possible.

A team of researchers at the University of Washington has designed a phone that does not have batteries. Instead, it draws a few microwatts of power from ambient radio signals or light.

The team eliminated the use of battery by converting analog sound signals that convey sounds into digital data.

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

The team demonstrated that the prototype phone can perform basic functions such as transmitting speech and data and receiving user input through buttons. Through Skype the team managed to receive incoming calls, dial out, and place callers on hold with the phone.

"We've built what we believe is the first functioning cellphone that consumes almost zero power," said Shyam Gollakota, an associate professor in the Paul G Allen School of Computer Science & Engineering at UW. "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."

The battery-free phone requires 3.5 microwatts of energy to perform some functions. It can operate using the power gathered from ambient radio signals transmitted by a base station up to 31ft away.

Using power from ambient light with a solar cell as small as a grain of rice, the phone was able to communicate with a base station that was 50ft away.

The team is working to improve the operating range and encrypting conversations to make them secure. It is also working to stream video over the battery-free phone and add visual display feature to it using low-power E-ink screens.

"You could imagine in the future that all cell towers or Wi-Fi routers could come with our base station technology embedded in it," said Vamsi Talla, a former UW electrical engineering doctoral student.

"And if every house has a Wi-Fi router in it, you could get battery-free cellphone coverage everywhere," added Talla.