Equivalent Circuit Model Considering Self-discharge for SOC Estimation of Vanadium Redox Flow Battery

The vanadium redox flow battery (VRFB) consists of the electrodes, pump, and membrane, but the pump is causative of self-discharge because it continuously operated the ions diffusion through membrane. Thus, the equivalent circuit of VRFB should consider the self-discharge to enhance the accuracy of the state of charge (SOC) estimation. This paper proposes the equivalent circuit model of VRFB, which is comprised of the basic RC ladder. In addition, to consider the self-discharge of VRFB the proposed equivalent circuit takes into account that the capacitance of the vanadium redox flow battery is changed by the charging and discharging cycles. The self-discharge test profile was proposed by applying the decreased capacity depending on cycles, and the derived amount of self-discharge was applied to the RC ladder based equivalent circuit model based on resistance. The parameters for the equivalent circuit model is extracted by the actual experimental data. The equivalent circuit model with the basic RC ladder is modeled by using MATLAB/Simulink. Based on the equivalent circuit model considering self-discharge, EKF (Extended Kalman filter) suitable for nonlinear system of vanadium redox flow battery was used for SOC estimation.