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Solution to problem of heating of metallic foreign objects during inductive charging of electric vehicles


The invention provides a simple solution to the "foreign object problem”, i.e., the heating of electrically conductive foreign objects in the magnetic field during inductive charging of electric vehicles.This solution presented enables an uninterrupted charging process.


In the search for environmentally friendly and at the same time fast mobility, an increasing number of car manufacturers is focusing on electromobility. However, for many end customers, the cumbersome and lengthy process of recharging the accumulator, among other things, still poses a barrier to the purchase of electric vehicles.

Conventional charging methods for electric vehicles use charging cables with above-ground charging stations. However, this design has definite disadvantages, particularly in terms of user-friendliness and spatial requirements. Moreover, the structure is prone to acts of vandalism. Inductive charging of the accumulators could be a solution to these problems.  In such systems, the primary coil - connected to the charging electronics - is embedded in or on the ground of the charging station. The secondary coil is located on the underbody of the vehicle. This means users do not have to make an electrical connection to charge the vehicle accumulator, but only have to park their vehicle above the primary coil to start the charging process.


There are no wireless charging stations available on the market, and so far, inductive charging of electric vehicle accumulators has only been implemented in prototypes at best. One of the challenges of inductive charging is to ensure that there are no electrically conductive foreign objects in the alternating magnetic field between the primary coil and the secondary coil, as these foreign objects can be heated by induced eddy currents and cause fires. Approaches to solving the so-called foreign object problem include using suitable detection devices for detecting foreign objects in the transmission path. If necessary, power transfer could then be interrupted. Examples are camera monitoring, light barriers, temperature monitoring, measurement of weight force and monitoring of electrical variables of the primary coil. However, camera and light systems quickly become dirty; other methods can be disturbed by weather or are not accurate enough, especially when it comes to detecting small objects. In any case, the energy transfer is interrupted upon detection of the foreign object, so the vehicle's accumulator is not charged until the object is removed.


A promising alternative to interrupting the charging process is to dissipate the heat from a foreign object. The wireless charging method for electric vehicles developed at the Institute for Electrical Energy Conversion (IEW) at the University of Stuttgart offers simple handling of the charging process using inductive charging technology and, at the same time, a safety-relevant solution to the problem of foreign objects. Adapted to technical and constructional requirements, the invention offers different solutions to dissipating heat, which can also be used in combination. The primary coil is ideally implemented as a system with a high copper and ferrite cross-section to keep heat generation low. If this is not possible due to external conditions, cold plates can be used for active heat dissipation. The primary coil is embedded in porous concrete with a sanded surface in or on the ground of the charging station. The high thermal conductivity, high heat capacity and low electrical conductivity of the concrete ensures that the coil heats up. If there is an electrically conductive foreign object in the alternating magnetic field, the heat generated is dissipated - depending on the design, either by the material surrounding the coil or by a separate thermally conductive layer on the parking space ground, which is separated from the embedding of the primary coil by a thermal barrier. Any risk of overheating of a foreign object is avoided at low cost and without any additional sensors required.


  • Cost-effective implementation without additional sensors required
  • Fewer interruptions of the charging proces

Fields of application

Inductive charging infrastructure for electric vehicles


M. Böttigheimer: Methodischer Entwurf von Systemen zur Gefahrenreduktion von metallischen Fremdkörpern beim induktiven Laden von Elektrofahrzeugen [Methodical design of systems for hazard reduction of metallic foreign objects during inductive charging of electric vehicles]

Dipl.-Ing. Emmerich Somlo
Ettlinger Straße 25
76137 Karlsruhe | Germany
Phone +49 721-79004-0
somlo(at) |
Development Status
Funktionsnachweis / TRL3
Patent Situation
DE 102019202553 A1 pending
Reference ID
Technologie-Lizenz-Büro GmbH has been entrusted with the exploitation of this technology and assists