If I had to guess, I\'d say that the 90 minute recharge time would be assuming the Airnergy\'s internal battery was already fully charged. I doubt very much you\'d pull enough energy from the ambient wifi in that period of time, but if it had been trickle charging itself for the last 2 days it makes sense.
David Fitch
Do you really care how long it takes to charge if your are on near constant trickle charge.
Gruph Norgle
In the EU the maximum legal Wifi transmission power is 100 milliwatts, however, on average 802.11 devices transmit at 50mw. Now in accordance with the inverse square law, at a distance of 5 meters, the power is reduced to 1 / (5^2) = 0.04 * 50mw = 2mw. Two milliwatts at 5 meters. Assuming they have a respectable electrodynamic induction efficiency of 75%, they will be able to convert 75% of that 2 milliwatts back into electricity: 1.5 milliwatts.
It takes 50 times that to power a single LED. This means that if their internal battery is 3 volts, 3000mAh it will take about 5000 hours to charge it to full capacity.
Brian R
RCA is making this? This is the type of snake oil nonsense that I would expect from some unheard of company. I am definitely in the believe it when I see it camp with this one! I understand the basic principles, but to me I think it would take days upon days to actually charge something.
Tapping energy from wireless signals should be quite feasible. I am sure the device does not depend only on the 2.4GHz of WiFi. In any case it is easier to make, and much more efficient, to have a broadband receiver that is picking up cellular (0.8-2.7 GHz)and other devices signals (possibly even TV signals in the ~1GHz).
There is quite a concentration of this kind of energy in an average urban space, probably enough to charge a battery over a period of, let say a day. The statement that it \"turns the WiFi energy into DC power \" is likely political. WiFi is operated by individuals and there is no claim for the energy emitted. Cellular and other transmissions might have other legal consequences.
However impossible it seems this must be akin to \"stealing\" power from overhead power cables - by whatever means. Wireless access points are provided for people to access the internet and the cost (including electricity that powers the access point) is borne by the customers of the location - be it a cafe or airport. All that will happen is that (food/ticket) prices will be increased to cover the increased electricity consumption as everyone charges their handhelds from the access point. I agree with Gruph Norgle who hit the nail on the head...
Gruph Norgle
@Deliverator: you don\'t seem to realise just how little energy can be harnessed. Even with 10 devices all within 5 meters (16.5 feet) it would take 21 days to charge a battery.
This sort of nonsense is why we haven\'t seen devices to \"harness the power of the AM and FM stations transmitting over the air\", or \"harness the power of the television broadcasts\"...
About the only thing comparable is a crystal radio, which has barely enough power from the radio wave signal to hear the signal over a earphone... barely. Definitely in the milli/microwatt range.
We are awash in radiofrequency waves from all over the city, nation and world, and the solar system. Just no practical way to capture it and convert it to useable, practical battery power.
Gruph Norgle
At the end of the day, they would make hundreds of times more power if the just attached a solar panel.
Brian R
@agulesin: Actually, the access point will transmit at 100mW no matter how many users are accessing it. If you had an airport terminal hotspot with 150 people using it to charge their batteries, the AP would not use any more electricity than it does at 3:00 AM when nobody is using it. This is the same idea as your local FM radio station. Every time a new listener tunes into their station, they do not have to turn up the RF transmit power to compensate. The transmitter does not get loaded down by the receivers, which is why the radio station has no idea how many people are listening unless they try to calculate based on the number of respondents to their call-in contests.
I do agree with you, however, that hotspot providers would likely charge an extra \"convenience fee\" or somesuch if they knew that tons of people were charging their batteries (even though it would not cost the provider a dime extra on their electric bill).