Inductive e-car charging emerges as alternative to conductive charging

June 10, 2011 // By Christoph Hammerschmidt
The charging interface for electric vehicles has been - and is - a subject to fierce discussion in the industry. AC and DC charging interfaces both have advantages and disadvantages. Now inductive charging is emerging as a surprise alternative.

All charging methods and interfaces for e-cars are characterized by tradeoffs between charging time, weight and installation space. Single-phase AC charging is convenient since it can be used in a private garage overnight. The relatively low charging current however calls for a long charging time of some eight hours. High-power (three-phase) AC achieves a faster charging cycle but is not available everywhere. Both low-power and high-power AC charging methods require an on-board rectifier and AC/DC converter - a relatively expensive, heavy and room-filling device. DC high-power charging would not have this disadvantage, but it requires a significant upfront investment into the charging infrastructure. Both AC and DC charging also require a standard interface with an equally standardized plug. Currently however several standard proposals are competing in various regional markets.

Inductive charging does not have these disadvantages. The interface is relatively simple, and since no cables and no high voltages are involved handling is safe and comfortable. Of course it also has drawbacks: A long charging time and the relatively poor efficiency. Despite these drawbacks, carmaker BMW and electrical giant Siemens in April introduced trials with an inductive charging system to the public. Located in the bottom of the car and the ground, the system's air wide gap between 8 and 15 cm however prevents efficiencies of more than 90 percent.

At the Automobil Elektronik Kongress in Ludwigsburg in the first week of June, automotive supplier Kostal GmbH now presented a technology with a much smaller air gap and thus higher efficiency. While the system still requires additional weight, space and costs compared to a DC system, it offers several benefits. Unlike the Siemens system which can be integrated into the road surface, in the Kostal system the magnet coils for the power transfer are placed behind the license plate; the system is charged through a wall box which typically would be installed in a home garage or a similar place.