RCA's Wifi "power harvester"


I'm having trouble believing that RCA's Airnergy, a "WiFi hotspot power harvester" unveiled at CES, can actually charge its internal battery from WiFi radio signals.

From OhGizmo!:

The Airnergy has a battery inside it, so you can just carry it around and as long as you're near some WiFi, it charges itself. Unlike a solar charger, it works at night and you can keep it in your pocket. Of course, proximity to the WiFi source and the number of WiFi sources is important, but at the rate it charges, if you have a home wireless network you could probably just leave anywhere in your house overnight and it would be pretty close to full in the morning.
A commenter on OhGizmo! offers the following:
Here's some math. Long story short, by my calculations, 100% efficiency and absorption at 5 feet away from a 100mW home router, (reasonable figures), it would take 34.5 years to charge that blackberry battery.

It's not a Dyson Sphere, so you only get the power that hits the antenna.

Surface of a sphere = 4pir^2, r = 60" (5 feet).

Surface area of a 5' sphere = 45,216 square inches.

The device appears about 2" x 3" = 6 square inches.

The device then picks up, best case, 0.000133 of the power out from the router, which is 100mW, so.. 0.0133mW

If you leave it there for 24 hours, 0.0318 mWh are stored.

According to Will's battery, it has ~4,000 mWh capacity.

So, it would take 12,579 days, or 34.5 years, to charge your blackberry battery once, presuming 100% absorption, no losses.

(BTW, What is that dent in the gadget about? Looks like someone poked it with an awl.)

RCA Airnergy Charger Harvests Electricity From WiFi Signals


  1. Could it, out of curiosity, be used to simply keep the item from dropping charge? I don’t really wanna do the math, but perhaps it might be worth it if it kept the battery from being drained while in use, thus extending the battery life.

    Admittedly though, I’d probably just carry my charger around.

  2. WiFi is for sissies. I’m going to design an electric car powered by local TV stations and cell phone towers.

  3. This is genius! I plan on powering my wifi router with one of these!!!

    Why didnt someone think of this before?

  4. There’s a bit more to this than the expanding sphere thing. Let’s assume your power received is -50 dBm (not unreasonable: verify on osx with the airport -I command). With a somewhat-nuts-but-not-impossible antenna gain of 40 dB (maybe that little dimple is related to this), you’re looking at 0.1 mW. Take into account that you may have a couple WAP’s in the area, and you may be looking at a usable amount of power.

  5. “Here is the really, really unbelievable part: RCA says that the USB charger will be available this summer for $40, and a battery with the WiFi harvesting technology will be available soon after”

    What’s unbelievable about that isn’t the price, its the fact that RCA is telling you that your $40 investment in “WiFi free energy” will eventually pay off in the “soon after”, when your external usb battery/charger will transform into a physics bending harvester of hotspot free energy vortices freeing you from the terrestrial based energy grid dependence. It’s really, in a way, a $40 investment in the future of energy independence which will one day be delivered by RCA.

  6. Hmm… not to be nitpicky, but I think most wireless antennas broadcast in a toroidal field, but I may be wrong.

  7. WiFi can be broadcast at 1 watt, not .1 watt. Assuming you’re in range of 10 WiFi hotspots (not uncommon in office buildings or apartments), that’s 1.33 mW for a charge time of only 126 days. That said, Lithium Ion batteries have a charge efficiency of at best 90% and a self-discharge rate of at best 5% per month, that’s 160 days.

    Of course, if you’re in a place with 10 WiFi hotspots within a 5 foot radius, chances are that there is a power outlet nearby too.

  8. The charging rate is dependent on how close you are to the WiFi hotspot, as the nice gent in the video explains. He never says *ho* close the thing was to the hotspot when the charged the Blackberry in their demo on the CES floor.

    I would not be surprised if it works within a few inches of the hotspot’s antenna, but for any distance over a foot, the signal will be too weak to make a dent in the battery’s charge (as the math-laden comment points out).

  9. I find this rather hard to believe.

    I mean, RCA is still in business? No frikkin way.

    Next you’ll be telling me that Abe Vigoda is still alive.

  10. > BTW, What is that dent in the gadget about?
    That is the orifice from which they pulled their power calculations.

    I call BS.

    Do the math:

    At 2.4GHz, 10m, assuming ideal dipole antennas I get 0.1% efficiency end to end. Some antenna gain would help, but to get 40dBi (@8) the antenna would have to get real big…like a sizable chunk of the 10m sphere. Fail.

    Plus the wifi transmitter isn’t transmitting most of the time – reducing avail power by the basestation duty cycle.

    So – good luck with that.

  11. Further, most WiFi radios that I’ve seen transmit at significantly less than 100mW. My recollection is it’s often less than one milliwatt.

  12. Ok my math sucks today – forgot the ^2
    Pr/Pt = Gr*Gt*(wavelength/(4*pi*R))^2
    Pr/Pt = 10^-8 = 0.000001% efficiency end to end. Fail^2.

  13. Note: WiFi devices are still FCC part 15, so limited to 100mW *max*. The energy to run this thing just isn’t there, even if you run a wire straight from the transmitter into it.

    They’d be more likely to make it work with a broadcast radio/TV station, at least they’re dumping much more energy into the air.

  14. The RCA brand is most likely available for licensing to whomever wants to produce cheap doohickeys with that name printed on it. Some okayish, some barely working, and some not working at all.

  15. Maybe it really does harvest broad spectrum RF energy including radio or TV station broadcasts, but they stick to saying WiFi because that’s an unregulated spectrum.

    [or maybe it just doesn’t work.]

  16. @21 – the FCC won’t stop you from receiving pretty much anything you want (except maybe other people’s cell phone calls)…it’s transmitting that’s tightly regulated.

    1. “@21 – the FCC won’t stop you from receiving pretty much anything you want (except maybe other people’s cell phone calls)…it’s transmitting that’s tightly regulated.”

      Yes, and it’s the transmissions that the device is supposed to collect. The thing that stops you collecting more energy from a signal than was put in at its source isn’t the FCC, it’s physics. You can’t collect 1W of power from a 0.1W transmitter without using magic.

  17. Perhapes someone should set up RCA and the “electromagnetic sensitivity” residence of Santa Fa on a bind date.

  18. This thing isn’t nearly as good as my perpetual energy machine, but it’s twice as good as my efficient Government.

  19. So many negative comments shooting this product down before anyone has had the chance to try it. How disappointing that speculation can hurt a product.

    I can’t believe that BoingBoing even posted this article in a Negative light! BB is supposed to be the the supporter of interesting and creative ideas, especially ones that are green and efficient [even if they are still prototypes!]. If it works or not, it is still a great idea for heading towards a greener future.

    And how about all you people test the item and then post your results… Keep your speculation and assumptions to yourselves unless you have factual observations.

    1. BoingBoing has always been critical of snake oil merchants, as anyone with half a brain should be.

      The maths on this one does not add up. Wifi simply is not transmitted at a power high enough to harvest it for charging a mobile phone, add in inefficiencies for power absorption and the whole thing is either fraud or a joke.

      Anonymous posters defending products are not always the most reliable source for fair and balanced opinions either.

    2. How disappointing that the public school system leaves many high school graduates unable to recognize when a product makes claims that defy the laws of physics!

      The relevant equations have already been posted in this thread. They’re not complicated. And I say this as a guy who’s failed algebra x times, where 2 < x < 5.

    3. And how about all you people test the item and then post your results… Keep your speculation and assumptions to yourselves unless you have factual observations.

      Sure, right after I fill my car’s tank with water just to see if it makes a viable substitute for gasoline.

  20. Is it possible this thing actually harvests power from ambient 60Hz AC fields? They are everywhere and often much stronger than wifi.

  21. If there was any validity to their claims about the spare energy being pumped into the air my cat, which is asleep atop my wifi router, would be baked in its fur.

    But it does give me an idea for a pocket sized device for harvesting energy from the tides. It uses the constantly changing gravity differential between the top and bottom of the device and the pull of the sun/moon/earth to generate power. If Anon #38 sends me $150 for shipping I’ll send him or her a demo unit.

  22. There is simply no way to gather enough energy from WLAN alone. Even if you would add the result of “harvesting” radio and tv signals, most cell phones will lose charge faster than this gadget could recharge it.

    Greetings, LX

  23. I remember a story about a guy who lived across the road from the BBC headquarters in Portland Place in London, where there was a humungous transmitter on the roof. Apparently, he managed to harvest enough energy from the BBC’s transmissions to power a bar heater – about 1kW. He was prosecuted – maybe for unlawful abstration of electricity. I’d love to be able to find a definitive source for this story.

  24. It’s useful to remember that “RCA” isn’t so much a company nowadays as a trademark that gets farmed out for a fee to various product manufacturers. According to this Wikipedia article, RCA the company basically ceased to exist in 1986.

    There are a number of old brands that now primarily exist as licensed names that other companies can put on their products. The familiar name can gives the products a perception of quality and reliability that might or might not exist. It depends on whether whoever now sells the licenses is more interested in maintaining the reputation of the brand or making a quick buck before the public catches on to a devalued brand.

  25. That’s not a dent. It’s a dimple. The dimple makes it all cutesy so you’ll forget the laws of physics.

  26. No no no no.

    You can’t plug your wifi router into just one of these. That would be perpetual motion, because you’d be producing the same amount of energy you’re consuming! You have to plug it into TWO of these, that way you have a surplus of energy. Duh.


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