Transfer function modelling for development of an CC-CV charging controller for Li-ion Battery

Abstract Lithium-ion batteries are widely used in modern power systems, for enhancing the flexibility of renewable energy sources through strategic charging and discharging. For smooth working and appropriate operation, it is crucial to employ an appropriate charging control strategy. The importance of proper charging control lies in the fact that it can improve the life of the battery and prevent it from early aging. This research work proposes a transfer function model-based SOC estimation for Constant Current-Constant Voltage (CC-CV) charging control. The proposed methodology consists of three phases in the first phase the transfer function model of the battery is identified and modelled. In the second phase, the SOC of the battery has to be estimated. Estimation of the SOC is computationally complex and requires a huge amount of memory for maintenance of the log of the SOC. Therefore, to alleviate these the paper proposes the use of a transfer function model-based SOC estimation that assesses the battery voltage response to pulse current at different SOCs. In this paper second order transfer function has been developed based on the 2-RC equivalent circuit model. The parameters considered for the estimation of SOC are pole, zero, and gain of the considered transfer function that is correlated to the SOC of the battery, and their value changes according to the change in the SOC of the battery. In the third phase, a control system has been designed to control the charging profile of the battery. The proposed system has been built by using the two-loop control technique that controls the duty ratio of the buck converter. A constant current-current voltage (CC-CV) strategy has been employed in the proposed model to charge lithium-ion batteries.