Wireless charging is a technology to transmit electromagnetic energy through air to charge a device, without any connecting wires. In 1890, Nikola Tesla was the first to conclude that power could be transferred between two objects wirelessly via an electromagnetic field. He experimented with transmitting power using inductive coupling which is one of the technologies that is being used today.

There are two main approaches to wireless charging that are used today.

Charging based on Inductive coupling works on the principle of electromagnetic induction. The principle of Electromagnetic Induction states that when a conductor is placed inside a changing magnetic field or a moving conductor is placed in a stationary magnetic field, a current is induced in the conductor due to the magnetic flux produced by the magnetic field.

In this case there are two induction coils, one in the base station (Transmitter Coil) or charger base and second in the phone/mobile device (Receiver Coil). When a current is passed through the transmitter coil in the base, it produces alternative magnetic field which eventually induces current in the receiver's coil. The receiver coil is connected to the battery charging circuit of the phone/device, which charges the battery. 

This technology usually provides a very high efficiency, sometimes even above 80% of all power is transferred to to receiver coil. However, the coils need to be very close to each other, typically at a distance of less than 7 mm. Also, the coils need to be perfectly aligned, as this can result in a significantly lower efficiency. Due to this only device can be charged at one time when using pure induction technology. Though you can use multiple coils, which can remove the alignment problem, but this still works best only to charge a single device.

Resonant inductive coupling or electro-dynamic induction is the wireless transmission of electrical energy between two magnetically coupled coils that are part of a resonant circuit tuned to resonate at a particular frequency. Resonance is a phenomenon in which a material naturally oscillates at a specific frequency, like tuning forks.

In electrical circuits we can create a resonant circuit using an Inductor and a Capacitor - This is called an LC circuit or a tank circuit. The circuit can act as an electrical resonator, an electrical analogue of a tuning fork, storing energy oscillating at the circuit's resonant frequency. In the case of wireless charging, two LC circuits that are configured to resonate in different devices; a transmitter coil in one device transmits electric power across an intervening space to a resonant receiver coil in another device which resonates at the same frequency.

Resonant transfer works by exciting the transmitter/base station coil with a current to make it oscillate at its resonant frequency. Once it starts resonating, any energy placed in the coil oscillates withing the circuit and then slowly dies out. Now if a second coil with the same resonant frequency is brought near it, the second coil also starts resonate and pick up any energy that was introduced in the first coil before it is lost. This energy is then used to charge the batter in the receiver circuit.

Charging based on resonant technologies typically has lower a efficiency, however has a number of advantages over Inductive coupling like a better charging range and the ability to charge multiple devices at the same time.

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