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Energy management systems (EMS) for vehicular applications control and optimize the power flow between electric consumers and power sources in the system. Control strategies of the EMS primarily aim at maximizing driving range and power of the vehicle with simultaneous consideration of physical degradation of the energy source due to the utilization. Previous papers consider hybrid energy storage systems consisting of batteries and Ultra Capacitors  or batteries and Fuel Cells . These systems are designed to gain best performance out of a high energy density source combined with a more powerful peak power device. Within the project 'e performance' supported by the German Ministry of Education and Research (BMBF) an electric vehicle powered by two lithium-ion battery packs of different capacity and voltage has been developed to make optimum use of available space in the car taking into account crash safety. Although using the same cell type in each battery pack, this topology results in a complex system with several components which must be managed via a data communication system (CAN bus). The EMS in this system controls the current flows of both packs independently by means of two DC-DC converters. The EMS acts as an intermediary between energy storage (battery management systems - BMS) and the drivetrain controller on the vehicle control unit (VCU). Main objective of the EMS is to provide all the propulsion power the driver requests and simultaneously minimize uneven battery degeneration. The usage of two DC-DC converters, two battery management systems and an on-board charger required the development of a new energy management system which interacts with the VCU. This paper describes the most important functions of the EMS and its interfaces to the BMS and the VCU. To validate the algorithms before integrating them into the first vehicle prototype, a detailed "Matlab/Simulink"-model has been created. The EMS presented in this paper is made to operate in - real-world sports car and fulfills therefore all automotive requirements including communication to all other relevant elements of the vehicle. However, this paper presents results from the simulations, because the car is scheduled to drive on the road from mid of September 2012 onwards. During the conference, data from the measurement in the vehicle will be presented additionally.