Correlation between capacity and impedance of lithium-ion cells during calendar and cycle life

Conventional capacity measurement techniques are time-consuming and thus expensive. But to know the capacity of battery units is necessary, e.g. to select most equal cells for battery pack assembly or to decide whether single units of an aged battery pack are worthy to be reused in a 2nd-life application. So, a quick and easy approach to refer to the actual capacity is of great technical and economic interest. In this paper, the correlation between capacity and impedance of lithium-ion cells during calendar and cycle life is analyzed and assessed, whether it can serve as a base for capacity quick tests. Therefore, new cells, cells aged in the laboratory and those out of two identical electric vehicles are characterized to yield a broad set of data. Results of this work imply the feasibility of correlation based capacity quick tests. However, parameterization of needed functional dependencies between capacity and impedance must be done with laboratory aging data similar to the practical use as a strong dependency of the correlation behavior from the operational and storage conditions is observed. Especially high temperature leads to strong deviation which could be linked to the layered structure of the solid electrolyte interphase.