A new method for determining SOH of lithium batteries using the real-part ratio of EIS specific frequency impedance

The State of Health (SOH) is a crucial parameter for evaluating the stability of battery operation. The SOH can be investigated via electrochemical impedance spectroscopy (EIS), which is useful for nondestructive battery testing and studying the working mechanism of batteries. The traditional EIS method used for estimating the SOH of lithium-ion batteries requires a long test time, complex circuit structure, complex calculation, or huge history data, making it inconvenient for practical applications. This paper presents a straightforward, convenient, and efficient method for estimating the SOH of Li-ion batteries, reducing the measurement time (3 min) compared with full-range EIS measurement (30 min) by a factor of 10. This method is based on the ratio between the real part of the zero-crossing of a Nyquist plot (impedance at 0.99 Hz in this paper) and the real part of the low-frequency inflection point of the Nyquist plot (impedance at 1976 Hz in this paper). This method is validated by different battery experiments. The obtained results show that this indicator does not significantly change with the battery charging capacity, and could be used to estimate the Li-ion battery SOH with an average relative error of 4.46 % through a second-order fitting function, which is of utmost importance for practical applications.