Underbody Charging Technology

Concept and functioning of the underbody charging system

In order to achieve a high range, large amounts of energy have to be transferred in short periods of time – similar to the standard refueling process. As the numbers of automated vehicles increase, it will be possible to execute a fully automated charging process from positioning to billing in the future.

Prototype successfully tested in the AULA project


© Fraunhofer IVI

At present, the market offers different charging systems for electric vehicles. If large energy amounts are to be transmitted, only conductive, i. e. contact-based systems (charging cable and plug), can be applied. These flexible solutions limit the system‘s charging capacity to approx. 150 to 200 kW and are therefore not suitable for fast-charging systems with higher performance.

The Fraunhofer IVI has developed a fully automated, reliable and safe charging system for this application scenario that overcomes the performance restrictions of conventional plug-and-cable combinations.

With the help of this new technology, it is possible to transmit one megawatt to electric passenger cars and two megawatts to electric trucks. All this is done fully automatically, safely and reliably without the involvement of a human operator. 

The system has

  • merely one interface for both fast and slow charging,
  • no charging capacity limitations and 
  • optimal operating comfort through full automation.

The use of charging cables will be no longer necessary.

Vehicle Interface

© Fraunhofer IVI

During the development process, special attention was paid to keeping the technology simple and easy to maintain. According to the directives of the automotive industry, the system‘s vehicle component needs to be very small, light and also cost-effective, given its prospective high production numbers. Additionally, the following requirements must be met: 

  • compatibility with DIN EN 61851-23-1
  • 2.5 liters installation space
  • 2.0 kg total weight
  • passive module
  • easy integration or retrofitting
  • compatibility with the infrastructure of both public and private areas

Public Area (Charging Station)

Because of the system‘s high transmittable charging capacity, the recharging process is comparable to conventional refueling in terms of duration.  

Therefore, suitable installation sites for the roadside charging interface are conventional gas stations. The charging actuator is embedded in a compact shaft system and placed 70 cm deep in the ground. Vehicles can easily drive over it for recharging. 10 to 20 charging processes per hour and contact system can be performed at this electric charging station. 

Key design aspects such as simplicity, robustness, durability and low maintenance  guarantee that the system will work smoothly over a period of several years:

  • 1 MW charging capacity for cars  (up to 3,5 t)
  • 600 km range within 5 minutes
  • 2 MW charging capacity for trucks
  • 100 km range within 3 minutes
  • fully automated charging process
  • integrated compensation of positioning tolerances
  • traversable by cars, load class D400


Private Area (Garage)

© Fraunhofer IVI

The Fraunhofer IVI also offers a cost-effective and simple solution for charging at home, for instance, in a garage. 

This system is simply laid down on the garage floor. Its maximum charging capacity is currently restricted by the typical limitations of home power connections:

  • recharging in non-public areas
  • 600 km range within 5 hours 
  • simple installation
  • Dimensions of garage system
  • 650 mm x 650 mm
  • Height 75 mm

The option of recharging in private areas allows the use of this newly developed charging technology even prior to a comprehensive, widespread introduction of the automated fast-charging system.

Safety and Standardization

© Fraunhofer IVI

Short charging periods require a high system voltage of up to 1250 VDC and charging currents of 2000 A and more. Therefore, the requirements towards the safety and reliability of the contact system are especially high.  

This affects contact protection, the prevention of electric arcs and unintended heating as well as a secure charging sequence control. In addition, basic safety regulations, for instance regarding slipping and getting caught, have to be followed. 

The system developers applied European and international safety and execution standards and norms regarding a clean and safe development process.

Fraunhofer IVI accompanies the development of the systems through active participation in the following standardization committees and interest groups at national and international level:

  • Founding member of the DKE working group 353.0.13 (Conductive Automatic Charging).
  • Member of the international mirror committee IEC-TC69-WG14 (EV supply devices with automated connection of a vehicle coupler)
  • Proposed solution »Automated Charging Device Underbody (ACD-U)« for private and public charging (Annex D of the future standard IEC 61518-26, currently under development)
  • Deputy head of the »ACD-U« working group of the Charging Interface Initiative e. V. (CharIn)
  • Founding member of the DKE working group 542.4.8 (ACD-U interface)
  • Member of the DKE working group 353.0.02 (Mega Charging System MCS)