Today I read an article about Airnergy, a product developed by RCA to recharge the portable device using Wi-Fi signal. I think the idea is pretty interesting because wireless power(not battery power) is a main research area in the next 20 years.
Here is the link to the article:
RCA Airnergy claims to charge gadgets using ambient WiFi signals
I think the comment from Gruph Norgle is pretty interesting:
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) * 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.
So, how long does an iPhone battery(3.7V, 1400mAh) get fully charged from zero?
Required power = 3.7V * 1400mAh = 5.18Wh Supplied power = 1.5mW = 0.0015W Hour to get fully charged = 5.18 /0.0015 = 3453.33 hours = 4.72 months