Wi-Fi Charging Works, But It Can't Really Power Your Phone

You can get power for devices just from a Wi-Fi signal, but it will likely never be enough to charge your smartphone.
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Yes, it's popular news right now. The story goes something like this:

The future is near! Soon you can just throw away those silly charging cables. You won't need them. In the future, your phone will be charged wirelessly just using existing Wi-Fi signals.

This sounds awesome, but it's not what you think. You are not going to be able to charge your smartphone with existing Wi-Fi routers (maybe some other things, as we reported, but not a smartphone). Most of these recent news stories are based on this arXiv paper titled Powering the Next Billion Devices with Wi-Fi. So, let's take a look at the paper and answer some questions (don't worry, I will do both the asking and the answering).

Is it possible to power devices with Wi-Fi?

Yes. In fact, the authors claim to run a small camera a distance of around 20 feet from a router. The camera then uses the Wi-Fi signal to charge a capacitor so that it can take one picture every 35 minutes. So, it's possible. You can run power a device over Wi-Fi.

That seems like a low power device. Would this work with a smartphone?

Technically, yes. This could charge a smartphone. However, in the example above the camera requires just 10.4 mJ to take one picture. If I use this and the time of 35 minutes, I get an average power of 4.95 x 10-6 Watts. Your phone probably requires around 1 Watt to run. Just to be clear, that is about a million times more power than what the camera uses.

What does distance have to do with this?

Let's imagine that the Wi-Fi router creates electromagnetic radiation uniformly in all directions. If this router produces electromagnetic waves with an output of 1 Watt, then this power has to be spread uniformly over an ever increasing sphere.


As you double the distance from the source, the surface area of this expanding sphere increases by a factor of 4 (since area is proportional to r squared). This means that the power has to decrease by a factor of 4. It's not an engineering problem, it's a physics problem. Oh sure, you might be able to fix this a little bit by using an antenna that does not radiate uniformly, but you still have decreasing power with increasing distance.

There is another problem --- the size of the device. If you have a relatively small smartphone (even the iPhone 6+ is small), it can only collect part of the electromagnetic radiation. A bigger phone would be able to collect more power, but who wants a giant phone?

Would this power over Wi-Fi cause trouble with Wi-Fi bandwidth?

That's the real question. It seems this device might just use parts of the wireless spectrum that aren't actively being used. However, what happens when more people use power over Wi-Fi and more people have routers near by? I think this could cause some problems.

So, you are saying this power over Wi-Fi will never work?

Nope. I didn't say that. I said this doesn't look like it will work for smartphones. Smartphones require way too much power to recharge or operate. This technology might be perfect for tiny sensors that need power (and are close to a Wi-Fi router). I can only think of two ways this would work with a phone. First, if you had some way of making a highly directional Wi-Fi that was beamed straight to the phone. In this case, you would be losing power to other stuff. Of course this also means that you would need some method for aiming this beam. The other way to make this work would be to decrease the power requirements of a phone by a factor of a million. Of course if that happened, you could run on your current battery for a million days.

Didn't Nikola Tesla invent wireless power?

Well, sort of. He showed that there were indeed ways to power things without wires. However, his method required some seriously big transmitters. Perhaps he didn't fully think through the whole wireless power plan. Telsa did not invent Wi-Fi and he did not invent power over Wi-Fi.

What about those wireless charging pads they make for phones? Are those the same thing?

They are similar, but not the same. Most of those charging pads use changes in magnetic fields to induce a current in a nearby loop of wire. Here is a more complete explanation (with demos) in case you are interested. But the biggest difference is for the charging pad to work, the device must be very close to the pad.

Ok, that's it. It looks like Wi-Fi charging is possible, but it's not likely to power or charge your phone.