Variable fractional order - A comprehensive evaluation indicator of lithium-ion batteries

This paper presents a novel indicator that captures frequency characteristics of variable fractional order in terms of the electrochemical impedance spectroscopy (EIS), which is adept at evaluating the performance of lithium-ion (Li-ion) batteries comprehensively. This model-free indicator is extracted geometrically from the capacitive resistance arc in EIS acquired off-line or on-line. Particularly, in order to acquire EIS on-line and allow for the practical applications of the proposed indicator, the fractional order impulse response time-invariant principle is applied to the digital implementation of EIS by using short-term and linear I/O data. A series of experiments on LiFePO4 cells and battery packs validate that the discussed indicator is closely related to state of charge (SOC), aging degree and discharge rate, which is presented as a comprehensive embodiment for states and performance of Li-ion batteries.