The system consists of two components: A primary coil integrated into a base plate which itself is placed beneath the vehicle, for instance in the floor of a garage or parking lot. This coil induces electric energy to the secondary coil in the car floor.
The arrangement of the coils, and consequently of the field pattern, is based on a design derived from their circular shape that offers a number of benefits such as a compact yet light construction as well as an effective spatial confinement of the magnetic field - a feature important to maintain high efficiency. The alternating magnetic field between the coils transmits the electric energy wirelessly at a power of up to 3.6 kW. BMW specifies the energy efficiency of this arrangement at 90%. The system aims at charging high-voltage batteries for plug-in hybrid and battery electric cars.
One of the design goals was achieving the shortest possible charging time. At a charging power of 3.6 kW, many plug-in hybrids can be charged completely in less than three hours. The BMW i8, the company's current technology flagship, can be charged in less than two hours. The i8 has, unlike its smaller sibling i3, a plug-in hybrid powertrain while the i3 is a battery electric car. To take higher battery capacities for purely electric vehicles into account, the future wireless charging technology standard offers the option to increase the charging power to 7kW which would make it possible to charge the batteries of the i3 over night.
To charge up a vehicle with the system, the car needs to be positioned exactly over the primary coil. A software in the head unit and a WiFi data link between charging station and vehicle supports the driver during this task. In addition, a smartphone app allows users to monitor the charging progress and informs about the remaining charging time.
Fig. 1: This is how the components for wireless charging