SOH analysis of Li-ion battery based on ECM parameters and broadband impedance measurements

The impedance of a Li-ion battery is an important parameter for the battery state-of-health (SOH) estimation. The dependency of the battery impedance to the SOH can be monitored by fitting an equivalent-circuit-model (ECM) to the impedance data and observe the changes in the ECM parameters. The ECM is typically fitted by the complex-nonlinear-least-squares (CNLS) algorithm which requires accurately chosen initial conditions for the parameters to guarantee the consistent performance of the algorithm. In order to use the ECM parameters for SOH estimation in practical applications, the impedance measurements should be fast and simple to implement to the battery system. This paper demonstrates the utilization of practical and fast pseudo-random-sequence (PRS) impedance measurements to the SOH analysis of a nickel manganese cobalt Li-ion battery by observing the variations in the ECM parameters. The measured impedances are fitted to the ECM by using the CNLS with adaptively obtained initial conditions. It is shown that the ECM parameters are changing as the battery capacity degrades. In addition, it is observed that some parameters are able to indicate a drastic reduction in the battery capacity and SOH.