The basic concept or technology behind wireless battery charging is that of inductive power transmission.
Inductive power transmission enables the power from an alternating current in one circuit to be coupled from one circuit into another.
As wires are not required for the transfer between circuits, inductive power transmission is a wire-less form of technology.
Inductive power transmission basics
As the name implies, an inductive power transmission system uses inductive coupling between two circuits as the basis of its operation. The system is effectively a two part transformer - the primary being contained within the power sourcing element, and the secondary being contained within the item in which the batteries are to be charged.
The primary and secondary coupled circuits are in the form of coils to increase the magnetic field of the circuits.
In this way, the transmitter coil has a current pass through it that generates a magnetic field. This is coupled to the secondary coil, and when there is a change in transmitter primary current, this induces a voltage in the secondary or receiver coil.
The voltage induced in the receiver or secondary coil can then be used to drive a battery charger or other circuitry as required.
Inductive power transmission efficiency
The efficiency of any inductive power transmission system is dependent upon a number of factors including the coupling, k between the inductors and their quality factor. In turn these are dependent upon a variety of other factors including:
- Inductor sizes: The ratio of diameters of the coils, D2/D1 has a direct impact on the coupling. It has an effect because for maximum coupling all the lines of magnetic flux should pass through the primary and couple into the secondary coil.
- Inductor shape: Again the shape of the coils will change the level of coupling of magnetic flux.
- Distance between coils: The distance between the two coils has a major effect on the efficiency of the inductive power transmission. As the coils move apart, the inductive coupling reduces rapidly as it is what is termed a near field effect. In practice efficiency levels of 90% and more can only be achieved if the distance to coil diameter ratio is less than about 0.1. Any greater than this and the efficiency of the inductive power transmission falls very rapidly.
- Coil resistance: The resistance in the primary and secondary coils will cause power to be dissipated as heat. This will be seen as a reduction in the Q or quality factor of the coils in the system
In view of the constraints upon the inductive power transmission systems if they are to achieve the maximum level of efficiency, the systems used often incorporate mats onto which the equipment to be charged is placed. This way the inductive coils are kept very close and the maximum coupling can be achieved.
Other schemes such as those used for electric toothbrushes also work well. However for these systems the space constraints within the mobile item to be charged are not so rigorous.
Each application will have its own constraints and requirements. Accordingly different mechanical arrangements are used in different areas.