How Wireless Mobile Chargers Work

There are several products on the market today that you can use to recharge a device just by setting the gadget down on a charging pad. Then, as if by magic, power transmits from the pad to the device. But it's not magic -- it's science!

Magnets, Electricity and Inductive Coupling

To understand how wireless power chargers work, we need to take a look at how magnetism and electricityare related. It's a relationship that makes possible hundreds of different types of electronic devices!

First, let's take an electromagnet. It's easy to make a simple electromagnet -- all you need is a battery, some insulated copper wire and an iron nail. Wrap the wire around the iron nail, leaving enough wire on either end to connect to the battery. Make sure each time you wrap around the nail that you're going in the same direction. The more coils you make around the nail, the stronger your electromagnet will be.

Once you've got your nail wrapped in insulated wire, you can connect the two ends of the wire to the terminals on a battery. Electricity flows through the coiled wire, generating a magnetic field along the nail. You can use the nail to pick up other nails through magnetism. If you switch the ends of the wire to the opposite terminals on the battery, you'll reverse the polarity of your electromagnet -- what was the north end of the magnet becomes the south end and vice versa.

If you assemble a second coil of wire and place it near the first, you can use the magnetic field from your electromagnet to create a flow of electrons in the second coil. If you hook that second coil of wire to a voltmeter, you can actually see the needle or readout change whenever you connect or disconnect the wires from the battery.

That's because exposing a coil of wire to a changing magnetic field can induce electricity to flow through that wire. The key is that the magnetic field must change to keep electricity flowing -- a stable magnetic field won't work as an inductor.

A battery provides electricity in a direct current -- the electricity always flows in the same direction. But if you hook up an electromagnetic to an alternating current -- a circuit in which electricity flows first in one direction and then the other many times per second -- you alternate the polarity of the electromagnet in time with the changes in the current's direction. That creates a constantly changing magnetic field -- the perfect condition for inducting electricity.

Transmitting Power

Most wireless mobile charging solutions rely on inductivecoupling. Here's a typical approach:

The charging station takes the form of a mat or other flat surface. Inside the mat are one or more inductive coupling coils. The mat itself is wired -- you have to plug it into a wall socket. Since theelectricity coming to your house is alternating current, the mat provides the electricity the coils need to generate a changing magnetic field.

Your mobile devices need a special case or attachment to take advantage of this magnetic field. Some manufacturers make devices with cases and electronics that facilitate inductive coupling -- the Palm Pre had this feature. But most manufacturers make equipment that still requires cables or wires to recharge. For these devices, you may need to use special sleeves -- each sleeve fits a particular model of a device. Or you may have to attach an adapter that plugs into your mobile device's charging port. The sleeve or adapter will have the matching coil to the surface's inductor coils.

Whether your device natively supports inductive coupling or requires a sleeve or adapter, your next step is to place the device on the charging surface. The inductor coils inside the mat generate the magnetic field that induces electricity inside your device, sleeve or adapter. This electricity then recharges your device's battery. Because there's no direct current passing between the mat and the device, it's perfectly safe to pick up a gadget when you're on the go.

Inductive coupling is useful but there are some downsides. The largest drawback is that it doesn't work over large distances. Placing a device a little too far to one side might mean that you aren't actually recharging the battery. Some wireless charging surfaces try to compensate for this by outlining where a device should fit on the surface or by creating raised areas that devices fit inside to make sure the coils are close enough together to work.

While inductive coupling is the most common approach to wireless chargers, it's not the only game in town. Let's look at some alternatives to induction.

Conductive Connections, Radio Transmissions and Wi-Fi?

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.

The Palm Pixi's design allowed you to charge the smartphone just by setting it on a charging station.

Courtesy Palm

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!