WASHINGTON (REUTERS) We've all been there - your mobile phone loses power just as you need to make an important call.

But now researchers have a developed a phone without any battery at all.

The no-frills device is constructed from commercially-available components and can receive and transmit voice calls and communicate with a custom-built base station.

It's powered entirely by harvesting power from radio signals, known as radio frequency waves.

"Ambient RF waves are all around us so, as an example, your FM station broadcasts radio waves, your AM stations do that, your TV stations, your cellphone towers. They all are transmitting RF waves," said Vamsi Talla, research associate at the University of Washington.

The team engineered the device to use far less power to make calls than regular phones using analogue rather than digital signals.

"If you look at a typical cellphone it actually consumes about hundreds of milliwatts of power while it's doing voice calls and it's impossible to actually make a device like that to be battery-free," said Talla.

For now, users of the phone have to wear headphones and hold down a button to switch between talking and listening, walkie-talkie-style.

But the developers plan more advanced prototypes with a low power screen for texting and even a basic camera.

"In the future every smartphone will come with a battery-free mode where you can at least make a voice call when your battery's dead," said Talla.

Eventually the team hopes to integrate their technology into standard cellphone masts or widely available wireless routers to achieve this battery-free mode for everyone.