Charging time estimation and study of charging behavior for automotive Li-ion battery cells using a Matlab/Simulink model

University essay from KTH/Skolan för elektro- och systemteknik (EES)

Abstract: An accurate estimation of the charging time of an automotive traction battery is possible only with the knowledge of different parameters of the battery and the vehicle. If this information is not available to the driver, the full time needed for charging of the battery may have to be assessed only from experience. A long route planning and estimation of required service life of the vehicle are therefore only roughly possible. Furthermore, with a better knowledge of estimated charging time, better management of public charging stations and better utilization of charging equipment can be achieved. An algorithm based on Matlab/Simulink model is made in the present thesis to estimate the charging time of a Li-ion battery pack which consists of 32 cells with 40 Ah each, as well as to investigate the impact of different cell balancing methods and different charging strategies on charging process. The theoretical background of the battery and charging modelling is investigated and different battery models are compared to get the best trade-off between the model accuracy and computation complexity. In the end, an electrical equivalent circuit model from reference [1], consists of a series resistor and two ZARC elements, is chosen to represent the battery cell. The parameters of the equivalent circuit are updated according to the SOC, current and temperature changes during the charging process. The whole simulation model of the algorithm consists of a charging controller (implementing the charging strategy), cell balancing logic controller, and cell balancing hardware simulation circuit and battery cell models. Different balancing criteria: based on SOC (with PWM drive) and based on terminal voltage (with/without advance) are implemented in the cell balancing logic controller, as well as different balancing windows, to investigate their impact on charging time. As for charging strategy, traditional CCCV is investigated, further investigation is conducted into improved CCCV method. The impact of initial SOC, charging rate and aging factor on charging behavior are investigated as well. Experiment results are validated by the comparison of the results with the ones got from a Hardware-in-the-loop simulation system.

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