Real-Time Estimation of the Instantaneous Open-Circuit Voltage and State-of-Charge of Rechargeable Batteries

This paper proposes a fast and accurate method for instantaneous open-circuit voltage (OCV) and state-of-charge (SOC) estimation in rechargeable batteries under both charging and discharging operations. The proposed method utilizes the ratio of the polarization resistance and ohmic resistance of the electrical equivalent circuit model of the battery as a key factor to reduce resistance variations caused by the thermal effects. The correlation between this resistance ratio and OCV enables real-time SOC determination using direct voltage measurements (with or without current measurements). The proposed method eliminates the need for (i) battery resting, (ii) an initial SOC value, and (iii) continuous sampling of historical data, which are typically required to minimize estimation errors of conventional techniques such as Kalman filter or observer-based methods. The proposed method has been applied successfully to four types of rechargeable batteries. Practical validation across varying thermal conditions has confirmed consistent estimation accuracy below 1.1% error throughout the complete charging and discharging cycles.