Multiparameters Model of the Initial SOC Considering the Relaxation Effect

To improve the accuracy of the initial state-of-charge (SOC 0 ) estimation, an enhanced SOC 0 model-based ampere-hour (Ah) model of a LiFePO 4 battery is presented for application in a battery energy storage system. It varies as a function of the open circuit voltage (OCV), rates, and relaxation effect. Here, the Thevenin equivalent circuit model is selected to model the LiFePO 4 battery, and its mathematic equations are deduced to determine the inherent trade-off between the time constant and main parameter of the battery, which can address the dilemma of the conventional SOC 0 method (OCV model) as the relaxation effect and how long the battery takes to reach a steady state. To illustrate advantages of the enhanced estimation model, a comparison analysis is performed on the enhanced model with the former model under the dynamic stress test. Furthermore, the experimental results indicate that the correction of SOC 0 can significantly decrease the estimation error. The enhanced SOC 0 model is expected to help determine operational parameters for batteries of variable power sources.