I'm not exaggerating about this. I don't mean to say that people in the 18th century were charging their cell phones with wireless charging. But in 1887, based on an article about Tesla's resonator in The World, Nicola Tesla discovered that electrical energy could be transmitted wirelessly. Based on this discovery, many companies are currently developing wireless phone charging devices that overcome the ease of charging smart phones without having to struggle to find cables.

How do wireless phone charging pad works?

1st Part : Wireless charging or Inductive charging works on the principle of the Faraday's law of induction which states that a changing magnetic flux can induce an electromotiveforce(EMF) in a coil. EMF is not a force but a potential measured in volts between the two open ends of the coil. The potential creates electric field which drives the current in any circuit connected to the EMF source or in our case the coil ends.

Faraday's law:

is Electromotiveforce of EMF

is magnetic flux.

is number of turns

EMF is proportional to the number of turns in the coil multiplied by the rate of change of effective magnetic flux generated by the magnetic source. In our case the flux source will be the charging station. Mutual coupling is ignored and the charging station is assumed to be very close to the device ,which means minimal flux leakage.

Faraday's law of Induction can also be understood with the following figure. The relative motion between the coil and the magnet changes the magnetic field strength over area or magnetic flux. This time varying flux induces voltage in the coil. Total EMF is the induced voltage in each turn multiplied by total number of turns in the coil for a tightly wounded coil.

The above mechanism can be used to charge a battery with voltage rating lower than the EMF. For example for a charger rating of 5 volts, 0.7 A charging a Li-ion battery of rating 3.7 volt 1650 mAh, i.e., the battery at a voltage of 3.7 volts can output 1.65 A for about an hour(or 6.11 Wh) it will take about 1 hr 45 mins to completely charge the battery from nil. Other factors like battery age and temperature also determines charging time.

As shown in figure below, the EMF can be rectified and used to charge a battery
, just like an AC adapter.

2nd Part : The charging station, as shown above, works on the principle of Ampère's circuital law using which one can determine the magnetic field associated with a current. By Controlling the current magnitude and flow direction one can produce magnetic fluctuations of desired magnitude and frequency. In the following figure the oscillator is tuned such that the rate of change of magnetic flux matches the resonating frequency of the receiver coil for maximum power transfer.