The allegedly-wireless allegedly-RCA "Airnegy" alleged charger

A reader writes:

You're probably getting about a million questions on this gizmo from CES, but do you think the RCA Airnegy WiFi charger is anywhere near remotely practical?

Airnegy charger

They claim it will "harvest" energy from 2.4GHz devices, like wireless phones and WiFi devices. They say it can charge a cell phone from 30% to full in 90 minutes on the CES floor, which is confusing because of all the wireless devices on the CES floor and the fact that many cell phones report full early to make their batteries look better. Since a Wi-Fi device operates at 100mW and that shrinks with the inverse-square law, wouldn't any Wi-Fi power be trivial?

Would this only be practical if you had a lot of overlapping Wi-Fi hotspots and/or a huge charger, or am I missing something? They're even claiming they can integrate this into batteries in the future.

If this is a scam or borderline useless, why is RCA promoting it? I could understand this kind of trash from a fly-by-night operation like all the fuel-pill pages, but I would think RCA would want to keep some of its reputation.


Yes, I think this has to be some sort of hoax. I ain't no RF physicist, but I don't think the numbers add up at all.

(I am, unsurprisingly, not alone.)

The output of the very small charger for my very low-powered mobile phone (a Motorola F3) is specified as 6.4V @ 200mA, which is 1.28 watts. The output of a standard Wi-Fi access point is, as you say, limited by the spec to 100 milliwatts. And, again as you say, the laws o' physics dictate that even if this thing contains a beautifully-engineered rectenna that hoovers up 90% of the 2.4GHz-ish RF energy that impinges upon it, it'll still collect far, far too little power to do anything very useful. For the same reason, it is difficult for a device the size of a canoe to harvest much energy from the wake of a passing ocean liner.

It's actually not quite as bad as you might think from a pure inverse-square law calculation, because the "impossible antennas" used in normal access points have a sort of inverse-hourglass-shaped radiation pattern, concentrating output around the antenna at the expense of output above and below it. If you're lined up with the radiation pattern of one of the larger "omnidirectional" Wi-Fi antennas, you could easily be getting three or four times as much power as you'd get if it were a real omnidirectional antenna.

But unless the Airnegy is squished right up next to the antenna so it covers, and near-totally absorbs, some relatively large fraction of the entire radiation pattern (and, of course, thereby makes devices in its "shadow" unable to see the AP any more...), then the energy it'll receive even from several out-of-specification half-watt Wi-Fi adapters will be extremely low. Never mind charging a phone - you wouldn't even be able to light an LED.

(A crystal radio can run on the RF energy from its own antenna, but that's in the microwatt range, at best.)

I suppose a device with some sort of broadband fractal antenna in it, that can suck up everything from 50Hz mains hum to high-gigahertz radar beams, might be more practical. But the Airnegy is said to be 2.4GHz-only.

Oh, and there doesn't seem to be any mention of this product on the RCA site. And although the Airnegy CES stand looks professional, the products themselves look like quick mock-ups to me. Look at this battery, for instance. It looks as if they put a construction-paper wrapper around a standard battery.

(I presume someone's paid to have the stand there, too, unless CES was having trouble filling the floor and let in hoaxers for free, like the funny fake ads that fill holes in newspaper classifieds.)

Note also that RCA is now, I think, one of those "zombie brands" that has been reduced to nothing but a logo that's slapped on random Chinese flea-market gadgets. So even if it actually is a "real RCA product", that doesn't mean much any more.

This also isn't a new idea. Here's a piece about a "prototype Nokia phone" that's supposed to somehow harvest five milliwatts from incident RF.

Can any readers who've got some of that fancy book-learnin' about that thar electrickery help me out, here?

(Somebody on that Boing Boing post busted out the Friis transmission equation.)

Has anything at all like this thing ever actually been made to work?

(And no, inductive chargers don't count!)

10 Responses to “The allegedly-wireless allegedly-RCA "Airnegy" alleged charger”

  1. frasera Says:

    yea, the lack of skepticism from many commenters/blogs is rather disturbing. As for the RCA brand, i guess they just whore that out to anyone now. All the major gawker sites have posted this thing without a word on how it doesn't pass the smell test:P

  2. Joshua Says:

    Recovering power from WiFi signals is, of course, entirely feasible from a theoretic standpoint. The problem is, as you say, collecting enough to be useful. The description I read suggested that it was using the absorbed RF energy to trickle-charge an on board battery.

    Or perhaps the on-board battery is the only source of power, which will trick the buyer into thinking it works for a couple of days, after which "RCA" will have already laughed its way to the bank.

    What's interesting, though, is that people really do seem to be working on real wireless power devices. Most of them, however, involve a dedicated transmitter/receiver pair (and fairly short, though still impressive, ranges), like this one:

  3. TwoHedWlf Says:

    My WRT54G apparently is specced to transmit at 20mw, so if I had that thing hard up against one of the antennas it would absorb at best about 5 mw. The article I read uses the example of charging a blackberry in 90 minutes. Which by my calculations at 5mw and perfect efficiency would take about 740 hours.

    I suspect it's like a friend of mine who bought a shake light off ebay. He turned it on and the led stayed lit for several days straight then no amount of shaking would make it work again. He emailed the seller and received a response of, "We sell two kinds of shakelights. One with batteries and one without." Why would I shake it then? FOR FUN?

  4. speedweasel Says:

    Its not often I visit Pharyngula and then Dan's blog to find the same front page news story!

    Actually, scratch that. It does happen a bit.

  5. corinoco Says:

    A comparison to solar cells:

    for 1 square meter of solar cell, with sunlight @ 1kW/sqm, with the most expensive cells you can reasonably buy (22% efficiency) and an average irradiance factor of 20%, you can expect 48.4W output. As long as the cells are sparkling clean and in a dust-free vacuum or clean room.

    For a device the size of a credit card (which is how big that thing looks)that equates to 0.2178W. Still assuming the 1000W/sqm source of course, which is as bright as the sun. If you source is in the milliwatt range however... you get 0.0002178W, or 0.2178mW of power from the antenna, assuming it is as efficient as the most expensive solar cells. Oh, and that isn't taking into account the inverse-square distance from the source, either.

    In an average room I think you would need some pretty expensive gear just to be able to measure the voltage this device produces, let alone get any useful power out of it. Unless it turns out to be mains powered, like those antennas you can buy that "improve" your tv reception by amplifying a bad signal.

    Real wireless power is of course possible - as long as you don't walk between the transmitter and receiver, or anywhere even remotely near the transmitter at all. Heck, you don't even want to be close to present-day mains power transmission lines without some form of protection.

    Induction probably doesn't count, as although it is technically wireless, the objects pretty much have to be touching. A "wireless" electric toothbrush won't charge on the other side of the bathroom, it won't even charge lying next to the charger - the induction coils must overlap.

    The inverse-square rule, plus the human body being an excellent receiving antenna for most forms of EM energy, is always going to work against 'beamed power' - you will always end up with a beam of energy that will work, but will also char-broil you if you get near it. As a wise man once said "Y'cannae change the laws'o'physics, Jim!"

  6. Shadowex3 Says:

    And corinoco answers the question I was going to ask. Those plugless charging shelves are probably induction based since they charge something that's mere millimeters away from the power source.

    Reminds me of the first batch of mousepad-charged wireless mice that came with the massive set of warnings on every piece telling you "DO NOT USE ON A METAL DESK". I'd imagine that these RCA chargers should come with something similar regarding universes with certain sets of laws of physics.

  7. Popup Says:

    Well, corinoco, your comparison with solar cells is a bit misleading. For photovoltaics the efficiency is indeed around 20%, but for microwave energy you can instead use rectennas (which Dan linked to) that are much more efficient. (up to 90%)

    Still, it doesn't change the fact that the 'RCA' thingy is a hoax.

  8. Daniel Rutter Says:

    you will always end up with a beam of energy that will work, but will also char-broil you if you get near it

    It depends on the area of the beam, though.

    The way they get around this problem for solar-power-satellite purposes (which are still largely theoretical, of course) is by transmitting the power as a microwave beam, but deliberately allowing the beam to diverge over a large area by the time it gets into the atmosphere. So you need a really big receiver station out in the near-equatorial boondocks or wherever, but the actual microwave irradiance per square metre is surprisingly low. No kids trespassing around the big weird wire thing or lost General Aviation pilots blundering into beam airspace will be barbecued.

    It's sort of like those supermassive black holes that're less dense than air.

  9. corinoco Says:

    Well, pilots might not be barbecued, but induced currents would cause hilarious antics in the avionics as they fly through the field.

    And, yes, supermassive black holes can actually be quite cosy places, especially for Heechee! I vaguely remember not believing that when I first read it, and then working out on my trusty C-64 that they could indeed live inside quite comfortably, if a little warm.

    @Popup - Rectenna effciency. I'm impressed, 75%-83% is pretty high, my engineering science and architecture experience makes me skeptical of any efficiency over 50%. Still, it's looking like a little over 0.5mW which is probably still useless.

    The "Do not use on metal desk" warning for induction chargers are because you will rapidly get a very hot desk and charger. This is how induction cooktops work, and you need to be careful where you put a metal cooking implement down when using them, because you can seriously burn yourself with a very hot knife.

  10. abfarrer Says:

    Look at this battery, for instance. It looks as if they put a construction-paper wrapper around a standard battery.

    yeah, that looks suspiciously like the battery from my Motorola RAZR with some stickers. now, that really makes me believe in the product, what with that battery being good for about half a day of standby time. (in an area with excellent signal, as long as you don't try to use the phone.)

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