The most widely used simulations of lithium-ion batteries are based on simple circuit diagrams und characteristics. Such models are only valid in specific ranges under specific operating conditions. For this reason typical challenges like the exact determination of state of charge (SOC) or state of health (SOH) cannot be simulated with the required accuracy. However, in order to be able to calculate realistic vehicle ranges for electric cars it is essential for ECUs to provide exact SOC calculations with respect to the current SOH. Otherwise calculations must be based on worst case assumptions in which the SOC shown is typically worse than the real SOC. The more exact the SOC values are, the more reliable is the possible range displayed for an electric car. Fraunhofer IWES has been working on the exact modeling of batteries for over twenty years. They have knowledge of almost any type of battery. In the last few years, Fraunhofer IWES has developed an electrochemical model for lithium-ion battery cells, the ISET-LIB. This model allows the design and parametrization of batteries and extremely realistic simulation of battery properties, enabling the user to simulate the internal behavior of the battery. All relevant quantities like voltage, temperature, influences of the aging of batteries, and so on can be simulated. A speed-optimized version of the model is available for fast HiL simulations. Because of the high level of details offered by the ISET-LIB, it was not possible to simulate the exact battery cells in the microsecond range. With over 20 years of experience in battery HiL-Simulators, MicroNova has developed a new generation of battery simulation cards that bring the resolution of a battery model down to step times in the range of a few microseconds. In order to achieve this, a number of innovations have been introduced. On the hardware side, all control of the cell simulation card has been made fully digital. In addition, new high-speed, high-precision hardware has been introduced to control cell voltages. This hardware functionality is combined with the Fraunhofer ISET-LIB in an innovative way. The ISET-LIB calculates the long-term behavior of battery cells very precisely every few milliseconds. Additionally, it delivers substitute parameters for the battery (e.g. R,L,C). These parameters are periodically updated in the battery simulation cards that model the short-time behavior. Thanks to this innovative approach, it is possible to achieve the simulation of a high-precision electrochemical battery model with a simulation step width at the microsecond level. Even applications like the simulation of RLC parameters for EIS (electrochemical impedance spectroscopy) are possible with this approach. This new and innovative combination of high-speed HiL and realistic battery simulation for the first time allows the development and testing of future battery functions like EIS and a realistic determination of vehicle ranges.
Autor: Matthias Puchta
Co-Autoren: Franz Dengler, MicroNova AG; Dr. rer. nat. Michael Schwalm, Fraunhofer IWES;