Wireless Charging Technology: What & How?

                 Wireless Charging (also known as " Inductive charging ") uses an electromagnetic field to transfer energy between two objects. This is usually done with a charging station. Energy is sent through inductive coupling to an electrical device, which then can use that energy to charge batteries or run the device.

Induction chargers typically use an induction coil to create an alternating electromagnetic field from within a charging base station, and a second induction coil in the portable device takes power from the electromagnetic field and converts it back into electrical current to charge the battery. The two induction coils in proximity combine to form an electrical transformer.Greater distances can be achieved when the inductive charging system uses resonant inductive coupling.

With the announcement of integrated wireless charging in theLumia 920, Nokia hopes it will be a significant part of the proposition to turn heads from Android and iPhone. But unlike previous examples like the Palm Pre's Touchstone charger, Nokia has joined HTC, Sony, Samsung and others by adopting "Qi", a proprietary interface standard created by the Wireless Power Consortium.

Qi, pronounced "chee", comes from the Chinese symbol meaning energy flow and is designed to provide energy to devices through magnetic induction. This is a similar solution to the system that charges electric toothbrushes. (It's also a valuable and legal two-letter word in Scrabble.)

In simple terms, the base station includes an induction coil that creates an alternating electromagnetic field. Meanwhile, a similar coil within the device is able to pick up this field, convert the energy into current and use it to charge the battery.

Inductive Coupling

Inductive coupling uses magnetic fields that are a natural part of current's movement through­ wire. Any time electrical current moves through a wire, it creates a circular magnetic field around the wire. Bending the wire into a coil amplifies the magnetic field. The more loops the coil makes, the bigger the field will be.

If you place a second coil of wire in the magnetic field you've created, the field can induce a current in the wire. This is essentially how a transformer works, and it's how an electric toothbrush recharges. It takes three basic steps:

1. Current from the wall outlet flows through a coil inside the charger, creating a magnetic field. In a transformer, this coil is called theprimary winding.
2. When you place your toothbrush in the charger, the magnetic field induces a current in another coil, or secondary winding, which connects to the battery.
3. This current recharges the battery.

You can use the same principle to recharge several devices at once. For example, the Splashpower recharging mat and Edison Electric's Powerdesk both use coils to create a magnetic field. Electronic devices use corresponding built-in or plug-in receivers to recharge while resting on the mat. These receivers contain compatible coils and the circuitry necessary to deliver electricity to devices' batteries.

A Splashpower mat uses induction to recharge multiple devices simultaneously

Wireless Electricity Transmission

Magnetic induction is a technology that you will probably remember from your physics classes at high school.

You need two coils, a transmitter coil and a receiver coil. An alternating current in the transmitter coil generates a magnetic field which induces a voltage in the receiver coil. This voltage can be used to power a mobile device or charge a battery.

The animation below shows how to build a wireless charging system using magnetic induction. Press > to start, and >> to move to the next slide. Take your time to watch the animation before you click the next slide.

You will see that products need more than coils and alternatining currents. For energy-efficient power transfer the phone must be able to shut down the transmitter when the battery is full. The phone, therefore, needs to send control signals to the charging station.

I recommend to look at slide 14 in particular. This slide show three different methods for aligning the transmitter- and receiver coils.

  A PPT showing how it actually works (click to play each slide)

Inductive Power Transmission

Dries van Wageningen and Eberhard Waffenschmidt, Philips Research
                    The basic principle of an inductively coupled power transfer system is shown in Figure 1. It consist of a transmitter coil L1 and a receiver coil L2. Both coils form a system of magnetically coupled inductors. An alternating current in the transmitter coil generates a magnetic field which induces a voltage in the receiver coil. This voltage can be used to power a mobile device or charge a battery.

                  The efficiency of the power transfer depends on the coupling (k) between the inductors and their quality (Q). (See also Figure of merit)
The coupling is determined by the distance between the inductors (z) and the relative size (D2 /D). The coupling is further determined by the shape of the coils and the angle between them (not shown).

An estimate of power consumption by wireless chargers.

POWER CONSUMPTION OF WIRED CHARGERS

Let’s first look at the power consumption of a classic mobile phone charger. These chargers are simple so-called “external power adapters”. A good source for data is the ENERGY STAR website. Here you will see that Energy Start compliant AC-DC adapters typically rate:

• Efficiency @ max load: 72% on average for 5 Watt adaptors
• Power consumption @ no load: 0.12W on average for 5 Watt adapters with a few exceptionally good adapters going down to 0.01 W

Suppose that you use the adapter for 1 hour per day, and that it remains plugged in for the rest of the day. That is not a good practice, but it is quite common to leave power adapters and cradles continuously connected to the mains.
You see that the total energy consumption is:

• charging: 1 hour * 2 W / 72% = 2.8 Wh (this assumes that 5 W charger will supply, on average, 2 W during a complete charging cycle)
• standby (no load): 23 hours * 0.12 W = 2.8 Wh

You see that standby power contributes significantly to the total energy consumption of a mobile phone charger.

WHAT ABOUT WIRELESS CHARGERS?

Our wireless chargers also contain an AC-DC power adapter. Let’s assume that is has the same efficiency (72%). Let’s also assumes that it has the same standby power (0.12 W). [footnote: Wireless chargers can have a much lower standby power, but this keeps the comparison easier.] The transfer efficiency of the wireless power link is typically 70%. And assume that the wireless charger replaces 2 wired chargers. The total energy consumption is:

• charging: 1 hours * 4 W / 72% / 70% = 7.9 Wh (we are now charging 2 devices simultaneously)
• standby (no load): 23 hours * 0.12 W = 2.8 Wh

HOW DOES THAT COMPARE WITH THE WIRED CHARGERS?

Total power consumption of two wired chargers: 2 * ( 2.8 + 2.8 ) = 11.2 Wh
Total power consumption of one wireless charger with two receivers: 7.9 + 2.8 = 10.7 Wh
You see that the total energy consumption is comparable. Although wireless transfer is obviously not as efficient as transport over a copper wire, wireless power transmitters saves standby power energy when the wireless transmitter replaces multiple external power adapters.

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Credits:-  http://www.wirelesspowerconsortium.com