State-of-charge and state-of-health estimation for lithium-ion batteries based on dynamic impedance technique

Abstract

This study proposes a dynamic impedance method to estimate the state of charge (SOC) and a projection method to estimate the state of health (SOH) of lithium-ion batteries. We defined changes in voltage over changes in current during charging and discharging processes as the dynamic impedance, which was then used to calculate SOC. The proposed methods do not require initial values, and dynamic impedance provides a more accurate reflection of the electrical characteristics of batteries compared to conventional estimation methods. This approach also enables calculations in real-time. The SOH of batteries degenerates with age, which also alters the characteristics of dynamic impedance. The proposed projection method determines the SOH according to the rate of change in dynamic impedance with respect to the SOC. We also developed a real-time SOC and SOH estimation system, combining a microcontroller and estimation methods with a battery management system (BMS). The BMS retrieves the data required for SOC and SOH estimation and includes various protection mechanisms designed to extend the lifespan of the battery. The hardware structure and the software procedure of the developed system are described in detail. We also present the results of experiments verifying the accuracy and feasibility of the proposed estimation methods.