Improved SOC estimation of lithium-ion batteries with novel SOC-OCV curve estimation method using equivalent circuit model

Abstract

The OCV-SOC curve is commonly used in SOC estimation and other battery management system (BMS) applications. Since the OCV indicates the magnitude of the charged battery voltage, the differs between estimated OCV-SOC curve and actual OCV-SOC curve are directly reflected in the accuracy of the SOC estimation and terminal voltage of model simulation. Thus, accurate OCVs are very needed but it is hard to obtain the suitable OCV-SOC curve for various current conditions. Furthermore, preliminary experiments for obtaining OCV-SOC curve spend lots of time due to repeats of the constant current discharge and taking enough rest time. In this paper, the new OCV-SOC curve estimation method is proposed to improve the accuracy of estimated SOCs and simulated terminal voltages of battery model without preliminary experiments. The proposed method uses equivalent circuit model (ECM) with parameter identification of recursive least square (RLS) algorithm, and is verified by experiments of various current conditions. Using the estimated OCV-SOC curve of proposed method, model accuracy is improved with lowered RMSE of simulated ECM terminal voltage and the SOCs were estimated less than 2% error.