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BMW, Daimler to advance inductive charging in EVs

10 Jul 2014  | Christoph Hammerschmidt

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BMW has announced a number of details regarding its development of inductive charging schemes for electric vehicles. In the medium term, the company plans to launch series production for the technology. The project is conducted together with competitor Daimler, in which they plan to provide a uniform charging technology for the garage at home.

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 3.6kW. BMW specifies the energy efficiency of this arrangement at 90 per cent. 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 3.6kW, many plug-in hybrids can be charged completely in less than three hours. The BMW i8, the company's 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 that would make it possible to charge the batteries of the i3 overnight.

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.

EV inductive charging

This is how the components for wireless charging are arranged in BMWs system (Stromversorgung = power supply; Sekundrspule = Secondary coil; Gleichrichter = rectifier, Energiespeicher = energy storage).

The system is weather-independent; it works also if the surface is covered with snow or water. The electric stray field is reduced to the minimum possible, stated BMW, without however specifying how strong the field is. The gap between primary and secondary coils is monitored constantly. If any foreign particle enters the gap, the charging process is stopped immediately, potentially saving the life of any curious pet cat.

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