The charging of batteries of Mobile device like Cell phones, Laptops, PDAs, Sensory Devices,Mp3,Cars ,Robots etc without using wires to connect to the power supply.
Almost all modern appliances one way or the other use electric current as energy source. Factories use electricity to run large machines, in our daily routine we encounter electric machines ranging from small earphones and watches to large machines like cars and trains. C:UsersBASHDesktopVector-icon-electronics.jpg
Some of the electrical appliances are mobile i.e. they can’t be constantly supplied power through electric wires from a power source, so batteries are used as a mobile source of electric energy , but they are like fuel tanks of any vehicle which needs to be refilled time after time. This can be done using a charger which is connected to a power source.
Now this constant need of recharging of its batteries reduces the mobility of the electric appliance or device. To make this electric appliance or device fully mobile we need to make the recharging process mobile i.e., the battery shall be rechargeable while on the move.
For this ‘mobile recharging process’, we need a mechanism to transfer electric charge to the battery via air independent of any wired medium.
Three types of techniques that can be used for wireless mobile charging are
Another idea was to use lasers to wirelessly charge mobile electrical objects. The limitations are lasers require an unobstructed line of sight and can also be dangerous to biological organisms. 
Before going into the details of the above stated techniques and other technicalities it would be a better idea to have a clear understanding of what is electric current, its transfer, what is a battery and how it works. Then we shall have some discussion about electromagnetic waves, the very basic principle on which our wireless charging techniques work.
1.1. I Definition
The Electric current can be defined as the flow of electric charges through a medium (gases, liquid or solid). Moving electrons carry it in a conductor such as a metal wire. In case of an electrolyte it is carried by ions and in plasma it is carried by both ions and electrons.
The SI unit for measuring the rate of flow of electric charge is the ampere, which is defined as, “charge flowing through some surface at the rate of one coulomb per second”. Electric current can be measured using a device called an ammeter.
Electric appliances function using the flow of electric charge either to get heat in case of electric irons, heaters and furnaces etc or to run motors (water pumps, air pumps, axles etc) or to represent and transmit data (telephone, computers etc).
1.2 Electric Battery
The scope of the discussion doesn’t allow us to venture into the details of how a battery functions or its composition so it is suffice to just give the reader a brief introduction.
1.2. I Definition
An electrical battery is combination of one or more electrochemical cells that convert chemical energy into electrical energy. 
1.2. II Types
There are two types of batteries used: primary batteries are designed to be used once and discarded normally called disposable batteries, and secondary batteries, which are designed to be recharged and used multiple times and are generally referred to as rechargeable batteries.
There are batteries of varying sizes from small ones to power wristwatches and miniature electronic devices to large ones the size of a large room to power submarines, super computers etc and to be used as alternate power source for houses, offices, factories and nuclear plants.
We are discussing the recharging of these secondary types of batteries.
The normal recharging process would be to attach the battery to a charger and plug the charger into a power source which is normally a socket. In of wireless mobile charging we need to transfer the electric charges from the power source to our battery charger without using wires as a medium for the flow of electric current or flow of electric charges. Now we either have to transfer the electric charges through air which is not possible as air is a bad conductor or devise a mechanism to produce electric current or flow of charges at the destination without the transfer of electric charges. The mechanisms used to produce the flow of charges at the destination without actually transferring electric charges to it are electromagnetic induction, resonance and radio waves. Now before going into the details of these mechanisms we shall understand the basics of electromagnetic waves the phenomenon of electromagnetic induction.
When Electric Current passes through a conductor, magnetic field is produced in the area surrounding that conductor.
In case of a copper wire conducting electric current the magnetic field can be visualized as a pattern of circular field lines surrounding the wire.
1.3. I Definition
The Alternating Current flowing through a wire produces alterations in the surrounding magnetic field resulting in the production of oscillating waves; these waves are called electromagnetic waves. The wire is twisted into a coil to create a strong magnetic field around it.
1.3. II Electromagnetic Induction
As the flow of electric current in a coil produces magnetic field in the area surrounding it inversely if another coil is brought in this surrounding region electric current starts flowing in this secondary coil, this phenomenon is called electromagnetic induction.
Nikola Tesla (10 July 1856 – 7 January 1943) was a genius way ahead of his times. An inventor, mechanical engineer, and electrical engineer he was considered eccentric by some while the greatest genius of all time by others. His greatest contribution was the promotion of Alternating Current (AC) for electric power supply. Although initially Edison’s Direct Current (DC) was considered better but eventually Alternating Current was adopted as the better option.
Tesla’s another interest was wireless transmission of electricity through air and ground using traverse waves for shorter and longitudinal waves for longer distances. As usual his idea didn’t get much serious appreciation and after his death little progress was made until the last decade.
The SI unit for measuring magnetic field or magnetic flux, the tesla, was named after him. 
Wireless Charging Techniques
The three charging techniques used for wirelessly charging a battery are:
3.1 Inductive Charging
In this method, an adapter with contact pins is attached to the back plate of the device. For charging the device is placed on a conductive charging pad which in turn is connected to a socket.
The electromagnetic field is used to transfer energy from the source to destination. Electric energy is sent from the charging station to the electrical device using inductive coupling which stores it in its batteries. Although the small gap between the two coils is small, still inductive charging is a kind of short-distance wireless energy transfer which can be enhanced by using larger coils.
Induction Chargers create an alternating magnetic field between the charging base station and the mobile electrical device. This is done using an induction coil. The power is taken from the electromagnetic field and converted back to electrical current and used to charge the battery by the induction coil in the mobile electrical device.
Greater distances can be achieved when the inductive charging system uses resonant inductive coupling. 
The Apple Inc has released and I-phone which uses charging pads to charge its battery instead of the typical wired chargers.
Transcutaneous energy transfer (TET) systems use induction charging and are used in artificial hearts and other surgically implanted devices.
Researchers at the Massachusetts Institute of Technology reported in 2006 that they had discovered an efficient way to transfer power between coils separated by a few meters. The team, led by Marin SoljaÄiÄ‡, theorized that the distance between the coils could be extended by adding resonance to the equation. In another MIT wireless power project, called WiTricity, curved coil and capacitive plates were used for better results.
A consortium of interested companies called the Wireless Power Consortium announced in August 2009 that they were nearing completion for a new industry standard for low-power Inductive charging.
Resonance charging is another wireless charging method used to charge batteries of electric devices that require large amounts of power, such as an electric car, robot, vacuum cleaner or laptop computer. This mechanism works for longer distances as compared to induction charging i.e. 3-5 meters.
When energy of a certain frequency or pith is applied resonance causes objects to reverberate (the phenomenon can be understood by the example of echo). In resonance charging, again two copper coils are used. First coil is attached to a power source and is called the sending unit. The second coil is attached to the device to be charged and is called the receiver. Both the coils are tuned to the same electromagnetic frequency. When objects of the same resonant frequency are placed close to one another, the energy produced can be transferred from one to the other. When an electromagnetic field is activated between these objects, the energy produced remains fixed rather than being dispersed in space. 
The energy produced between objects with long-lived resonances has little to no effect on the environment or biological organisms, making this method of charging much safer as compared to using laser.
An example of resonance charging used to deliver power to the battery of an electric car can be: The garage and car would both be equipped with copper coils. The garage would contain the transmitter, while the car would be equipped with the receiver. The transmitter supplies the room with a non-radioactive magnetic field. This field will adjust the power transfer, producing a powerful correlation between the sending and receiving unit and, as a result, delivering a charge to the car’s battery.
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This power transfer can be done between the sending and the receiving units effectively without being affected by the shape and dimensions of the enclosed space either an office or house. This is not affected by any electromagnetic waves of other frequencies and in turn also doesn’t affect any electronic device in its vicinity that is operating on another frequency.
Resonance can also be used for wireless power rather than wireless charging. For example, a laptop equipped with a receiving unit and in the same room as the sending unit could receive an automatic charge as a constant flow of energy and not require the use of its own battery, but that will reduce its mobility as the laptop couldn’t be used while outside the range of resonance power provider.
The third method used for wireless charging is Radio charging, but it can only transfer small amount of energy and is used to power small batteries. This can be used to wirelessly charge electronic devices with low power requirements like watches, hearing aids, medical implants, cell phones, wireless keyboard and mice.
Cellular telephones already use radio waves for transmission and the same radio waves can be used to charge their batteries or power them. Radio waves are used for radio, television transmission and Wi-Fi signals. . A radio wave, once transmitted, propagates in all directions until it reaches an antenna tuned to the proper frequency to receive it. For electrical transmission the same principle is utilized.
To understand how radio charging is done, suppose we have a wirelessly charged watch. The transmitting end or source sends a low power radio wave at a fixed frequency and whenever a wristwatch having radio wave power receiver is set to the same frequency as the source’s transmitting frequency comes within its range, the radio wave power receiver powers the wrist watch and starts charging its battery.
As discussed earlier the radio charging is effective only for devices with low power requirements. Electrical devices like laptops and vacuum cleaners require more power than radio waves can normally deliver.
Another limitation is effective range of radio charging as the distance from the source increases the power of the radio signal drops. So for a better charging the receiver must receive a powerful radio signal and the closer it is to the source the better the reception will be.
Advantages and Disadvantages
As compared to wired charging there is almost no risk of electric shocks as there are no exposed conductors. Resonance and radio charging makes it ideal charging method in cases where water impermeability is required. It can be used for implanted devices which require frequent or constant charging. It can also be used to charge toothbrushes and shavers which are used near water and can be used even in it.
Wireless mobile charging increases the mobility of the device and makes their use convenient.
As compared to wired charging wireless charging is less efficient in terms power being transferred with respect to time. It also causes increased resistive heat and devices using lower frequencies make the charging process more slow.
The manufacturing complexity and expenditure is very high as compared to wired chargers. Thus decreasing the feasibility and resulting in lack of interest from investors.
Wireless charging of devices with large batteries is still not being implemented and requires a lot more time to be able to do so.
However new technological advancements like use of ultra thin coils and higher frequencies have resulted into efficient charging and receiving units that can be integrated into electronic devices and batteries with minute alterations.
Several health experts have shown reservations about the effects of large scale use electromagnetic and radio waves but no credible research has been conducted in this regard and safety limits are already in place.
Although remarkable and very efficient there has been very little development or breakthrough made in the field of Wireless Transfer of Electricity since the idea was first floated by Nikola Tesla around 100 years ago. A world free of electric cables is a distant dream yet hope for a future that would be echo friendly and free of hazardous fuels like carbon and nuclear keeps us going.
Fortunately in the last decade a great deal of attention has been given to the Concept of Wireless Energy Transfer which has resulted in a lot of funding into the projects once considered impractical and eccentricities of a mad man when Tesla proposed them .
Although still not as efficient as wired charging at present, in future they efficiency of wirelessly charging devices would hopefully be enhanced and wired power supply may altogether be discarded.
The increased demand of independence of mobility has made possible great inventions, as necessity is the mother of all inventions the time has come for best brains of our era to produce efficient means of Wireless Electric Energy Transfer.
The future is bright and full of surprises.
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