Real-time internal temperature estimation of commercial Li-ion batteries using online impedance measurements

Thermal monitoring of Li-ion batteries, particularly when a large number of batteries are interconnected, is a mandatory requirement to ensure their safety, performance and longevity. In this paper, a real-time internal temperature estimator for Li-ion polymer batteries is introduced. Firstly, the influence of temperature on the impedance characteristics of the battery at various frequencies was explored. Next, the influence of state of charge (SoC) and state of health (SoH) on the different parts of impedance were examined. The insights learned were used to develop a temperature estimation model based on a single-point frequency measurement at 200 Hz. All impedance measurements were performed online while the cell was under load. The impedance at 200 Hz demonstrated high sensitivity to changes in temperature and low sensitivity to SoC and SoH. The model was validated experimentally using data collected from 9 separate cells cycled over 100 times each. The average RMSE of the estimated temperature across all data sets was 1.41 °C, confirming the accuracy of the model.