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Found recently: A device that supposedly charges USB devices from ambient WiFi-fields. Interesting, there should be enough energy in Wifi fields for that? Let’s do the math: In Europe, Wifi devices are limited to 100 mW transmission power (EIRP). Looking at that device, I would suggest a surface of about 10 cm x 10 cm is not a bad estimate. Given a distance from the access point of about 1 m, it can harvest about of that, so about 1 mW. That’s not much… and even given several sources, I think it would be very optimistic to assume that they are all as close a 1 m. And then, the power received drops quadratically.

At this rate, to charge a normal mobile phone battery (typically several Wh) takes thousands of hours, assuming 100% efficiency and continuous transmission. The only way to really get down to a number where you could seriously consider it would be putting that device just flat on top of your access point, which can giv you 50% of its transmission power at best and would make it 20 hours.

Let’s just say that is incredibly stupid. The only comfort you’ve won is not having to plug it in (remember you still have to plug in your mobile phone into the Airnergy, AND you would have to put that device directly on top of the access point for a day). But it kind of defeats the point of the access point if you put something directly on top that will block all radiation. So it is basically mostly running to charge this battery, which is incredibly inefficient. Something must be wrong, right?

Now, in the article, they claim:

At CES, the device’s battery, which I believe was precharged with Wi-Fi power, was able to charge a BlackBerry from 30% power to full power in about 90 minutes.

Well, that was basically a nice PR stunt. The 90 minute number is obviously completely meaningless, as that was from the Airnergy’s battery. I would think it is quite improbable that it was actually charged using Wifi. And even if it was, then certainly in a way quite different from what the author expects …

A further comment: Even in inside, closed rooms, solar cells would give you a much better yield than that. And still, most people don’t use this to charge their phones (devices exist, though …). Guess why …

Well, if you ever come across these, don’t buy.

This entry was posted on Wednesday, January 20th, 2010 at 7:08 pm and is filed under Computers. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.