
An RF-powered cordless phone - jonbaer
https://www.wired.com/story/this-cell-phone-can-make-calls-even-without-a-battery
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userbinator
Neither the title of this item (currently "A cell phone powered by ambient
light") nor the article's ("This Cell Phone Can Make Calls Even Without a
Battery") is really accurate.

 _Because the phone relies on those signals for its energy harvesting, it has
a range of just 15 meters from the basestation._

This is neither a cellphone nor solar-powered. It's an RF-powered cordless
phone. Interesting nonetheless.

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TeMPOraL
Well, _technically_ RF-harvesting phone _is_ powered by "ambient light". The
title says nothing about solar, or _visible_ light.

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occamrazor
The word "light" denotes EM radiation in the visible spectrum or in the near
IR/UV only, not generic EM radiation.

~~~
TeMPOraL
To normal folks, maybe, but in technical language it can refer to the entire
EM spectrum (as there's no meaningful difference between "visible" and
"invisible" EM radiation for anything other than human eyes).

EDIT:

I looked up the spectrum PV cells tend to respond to and some sources on the
usual meaning of the word "light", and I admit my original comment was me
being pedantic about the more obscure usage of the word. Sorry :(.

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ClassyJacket
...which makes it pretty meaningful to humans.

Also consider what of the sun's radiation makes it to the surface of the
Earth.

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TeMPOraL
Fair enough.

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donquichotte
An IPhone battery has a capacity of roughly 10Wh. The energy density of the
sun is approx. 1kW/m2 on earth. Let's assume a solar cell with 20% efficiency,
so 200W/m2. To charge the iPhone battery in 4h you would need a solar panel
with an area of 11cmx11cm. So the phone is probably not very powerful.
Certainly it's power draw is nowhere near current smartphones. (It's mentioned
in TFA, 800mW when calling, tens of microwatts when on standby)

~~~
TeMPOraL
That... sounds too good to be true. 11x11cm panel is basically a pocket one;
hell, my S7 has surface area (one side) of ~80% of that. And 4h for charging a
smartphone is a typical time.

So it sounds like not only this should be very useful according to those
numbers, but would also imply that solar chargers for smartphones are not
complete bullshit fake non-product I always assumed they were?

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kqr
Huh, I guess you're right.

My phone has a battery of around 12 Wh and lasting maybe 30 h on a full
charge, that's a power consumption of 0.4 W on average. With a surface area of
about 70 cm², we get about 60 W/m².

Phone eats 60 W/m².

Sun provides 1000 W/m².

Okay, let's cut that into a fourth because, you know, clouds and nights and
stuff. Retake.

Phone eats 60 W/m².

Sun provides 250 W/m².

Essentially, if you find a solar panel with 25% efficiency and the right size,
you can slap that on the back of your phone for a very tiny size increase, and
you now have a phone that powers itself as long as you put it down in the sun
instead of in your pockets when you don't use it!

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morsch
250 W/m2 is still rather optimistic -- though not absurdly so!

Obviously it varies a lot depending on where you are. For example, in Germany
the total sunshine you have to work with is approximately 1000 kWh/year[1], or
115W/m2 on average. I think that still assumes you find a way to perfectly
align your phone to be perpendicular to the sun's rays; otherwise you get less
energy still.

[1]
[http://www.solar.lucycity.de/index.php/sonnenenergie/9-sonne...](http://www.solar.lucycity.de/index.php/sonnenenergie/9-sonnenstrahlung)
I think the technical term may be solar irradiance, here's a global map:
[https://en.wikipedia.org/wiki/Solar_irradiance#/media/File:S...](https://en.wikipedia.org/wiki/Solar_irradiance#/media/File:SolarGIS-
Solar-map-World-map-en.png)

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TeMPOraL
Two observations:

1\. This sounds very useful both as a backup capacity to make calls when your
battery died; a smartphone could run in a restricted, super-power-saving mode
off that. Could be handy in cities, where there's _lots_ of RF background.

2\. Energy that can be harvested this way from human-made RF is energy wasted.
I expect less and less of such background RF to be enabled as time goes on and
people figure out more tricks to do hyper advanced magic with beamforming.

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jimmies
>1\. This sounds very useful both as a backup capacity to make calls when your
battery died; a smartphone could run in a restricted, super-power-saving mode
off that. Could be handy in cities, where there's lots of RF background.

Just make 5% battery the new 0% and whatever super-power-saving mode magic you
have to run off that 5%. I think that might make more sense in terms of power
envelope than to bother with all this stuff.

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relyks
Here's a link to the research paper for those who are interested: "Ambient
Backscatter: Wireless Communication Out of Thin Air"
([http://abc.cs.washington.edu/files/comm153-liu.pdf](http://abc.cs.washington.edu/files/comm153-liu.pdf))

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gwbas1c
It looks like the power limitations of running from radio waves are limiting.
What about a battery or phone that charges from radio waves? It would give the
phone a lot more power to operate, because the phone itself doesn't run 24/7.

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TheAdamist
from the article, harvesting RF generates tens of microwatts at best, which
probably isn't enough to even operate the charging circuitry much less
actually charge anything.

