Li-ion cells internal temperature estimation using medium-frequency measurements of impedance argument

The thermal runaway of Lithium-ion batteries can result in severe accidents hindering their adoption and is, therefore, a cause for concern. Monitoring the temperature of the cells in a battery is one of the most effective methods to anticipate a thermal runaway. Since readings from a temperature sensor on the surface of a cell may lag behind its true internal temperature, they cannot be used to predict the advent of thermal runaway. Electrochemical Impedance Spectroscopy (EIS) can be used to provide insights into the internal properties of the cell. This work builds upon a previous study that proposed a method to use the phase of the impedance of the cell to estimate its internal temperature, where the Distribution of Relaxation Times (DRT) technique was used to infer the frequency at which this impedance measurement should be carried out. In this paper, a comprehensive experimental validation of the proposed methodology is performed on a set of NMC Lithium-ion cells, by placing a temperature sensor inside them and comparing the estimated and surface temperature under various electrical and thermal loads. Furthermore, an investigation on the influence of SoC on the estimated temperature is also performed.