Another approach to wirelessly recharging mobile devices uses a more direct route. Conductive recharging mats create a direct electric circuit between a mobile device and a charging surface. The charging device's surface has strips of conductive metal on it. When a device with corresponding electrical contacts touches these strips of metal, electricity flows into the device.
For this to work, the device must have the contacts incorporated into its case or a special sleeve that has the contacts on it. You snap your device into the appropriate sleeve -- each model and type of device needs its own -- and place the sleeved device on the right part of the surface, ensuring contact. This creates a circuit and charges your device.
You have to make sure the contacts on the sleeve match up with the conductive strips on the chargers surface or you won't create a circuit. But conductive strips can be more efficient than inductive coupling, which according to the Wireless Power Consortium averages between 50 and 70 percent efficiency [source: Higginbotham]. That means at least 30 percent of the power needed to run the charging station goes to waste, even in an efficient inductive coupling system.
While radio transmission of power isn't efficient, it's possible to design a charging station that converts radio frequency waves into direct current electricity. But unless you have a large antenna and a particularly powerful broadcast source, you won't harvest much electricity from the ambient signals around you.
In 2010, RCA announced the company was developing a wireless charging station that could harness WiFi signals and convert them into electricity. If such a device could work, it could be a great resource in any place that had a WiFi hotspot. But mathematically, such a device isn't practical. WiFi routers only emit only a tiny amount of energy -- often around one-tenth of a watt. A converter would only capture a fraction of that energy, which means charging something like a battery just once could take decades if you depend entirely on WiFi signals to supply the power.
Why only a tiny fraction? It's because energy emissions obey the inverse square law. This is a law that states any point source that spreads out evenly in all directions -- such as a radio wave -- will reduce in intensity relative to the distance from the source. When you start with a low-power energy emission and then move away from the source ,the intensity drops away quickly.
With so many options for wireless chargers, we may be close to saying farewell to tangled cables and proprietary plugs -- and good riddance!