High Frequency Impedance of Li-ion Batteries

The development of electrically driven cars focuses primarily on the battery since this is the restraining factor for driving range and performance. In addition to this DC characteristic, the battery also has a high-frequency behaviour which has to be evaluated in order to ensure proper functionality, system stability, supply voltage quality and EMC. This paper explains three different methods to measure the frequency-dependent impedance of actual high-voltage batteries up to 10MHz. For comparison and evaluation of the techniques presented, a reference battery is measured and the results are compared. Additionally, a method to theoretically calculate the particular impedance parameters of the measured battery and the result for the entire battery is shown. The results derived are used in a system simulation in order to validate the measured values in contrast to the theoretical ones. Finally, the measurement techniques are compared; their advantages, disadvantages as well as the preferred scope of application are shown, and the results of an additional measurement with a larger battery is performed and compared-.