Battery-Free Phone Works But Isn’t Practical

Credit: Mark Stone/University of Washington

A new cellphone design works without any battery. But it has some major drawbacks and would require some major infrastructure work to take off.

The phone is the work of University of Washington researchers. Although it doesn’t have a touchscreen or any smartphone apps, it does use Skype technology for voice calls.

The energy use of the phone is 3.5 microwatts. To put that into context, the rate at which it draws power would be enough for an iPhone battery to run continuously for just under 250 years. (Special thanks to Wolfram Alpha for not only facilitating that calculation, but for helpfully adding that this is 1.3 times the “approximate time one would have to lie supine before a bird would poop in one’s mouth.”)

The big power-saving is getting rid of the step that turns analog signals into digital data. Instead it works solely with analog radio signals. When receiving, it turns the signal into vibrations: in the current design, the user hears them through plug-in headphones as there isn’t enough power for a traditional speaker to be audible.

When transmitting, the phone doesn’t create a signal itself, but rather modifies a reflected signal from a base station, which then deduces the audio from the modifications. For the testing at least, the actual calls were routed via Skype, with the Internet connection in the base station.

The small power that is used by the phone can come either from harnessing the power of ambient radio signals, or from a tiny solar cell. This was one of the main challenges of the design as the power has to be constant throughout a conversation.

While technically impressive, it’s far from a practical solution right now. The main limitation is that the base station would have to be built into cell towers or wireless routers. The researchers argue that if future equipment had the technology built in from the start, battery free cellphones could work anywhere that normal wireless phones do. While that’s true, it doesn’t really address the business model or the incentives that router manufacturers and cellphone networks would have to adopt the technology.

Another limitation is that the current design requires the user to flick a switch between receiving and transmitting modes during a conversation, walkie-talkie style.

In the testing, the base station had to be within 31 or 50 feet of the phone, depending on the power source. However, the researcher stress this limitation is mainly because of the frequencies they had to use in testing, noting that if the phone operated on traditional cellphone frequencies the range would be considerably longer.


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