Bi-directional Charging System Design for a set of Li-ion Batteries Located at Angstrom Laboratory Campus of Uppsala University

Abstract: In this study, onboard chargers for EVs are investigated and a design of bi-directional onboard chargers is proposed and simulated. The goal of the charger is to be built in the future to be used in the test setup at Uppsala University. The charger consists of two stages: a power factor correction (PFC) converter, which converts AC voltages and currents from the grid side to DC while maintaining a unity power factor, and a bi-directional buck-boost converter, which regulates the charging and discharging current of the battery. The model was built using MATLAB/SIMULINK and the d-q synchronous reference frame was utilized to implement the current controller of the PFC converter, while the bi-directional buck-boost current controller was constructed using DC pulse width modulation. The Proportional and Integral gains were tuned using the MATLAB single input and single output tool (sisotool). The converter's topologies, structure, and corresponding mathematical model were investigated, and the charger was simulated and tested for charging and discharging modes. The battery voltage, current, and state of charge were monitored during all modes of operation to evaluate the performance of the buck-boost controller, and the functionality of the PFC controller and filter was tested by measuring the currents and voltages on the AC side. The charging and discharging efficiencies were mapped under various battery voltages and current sets to determine the performance of the charger under different operating conditions. The charger demonstrated excellent performance during charging and discharging modes and recommendations for future work to improve the efficiency and performance of bi-directional charging systems were provided.