Extraction of Single Cell Impedance from within a Battery Pack by Virtual De-Embedding: A Proof of Concept

Models for the simulation of battery pack impedance are usually composed of models for the individual cells which the pack is made of, linked with a description of cell-to-cell and cell-to-housing coupling. Thus, conventional battery pack modeling requires knowledge of the cell first, which is usually obtained by measurement on single cells. In this work, a solution to the inverse problem is described, i.e. measurement of the pack is available and impedance of the cells within shall be derived. Therefore, the pack's impedance needs to be partitioned into the cells' 'internal' impedances and exterior coupling effects, like mutual inductance. Proposed method employs 3D simulation of the battery pack with surrogate cell models. Measurement data and simulation model are then combined to find individual cell impedances by fitting the simulated pack impedance to the measured. For validation of the approach, single cell impedances obtained by virtual de-embedding from different measurement setups are compared and related to reference results from literature. Considered frequencies range from 9 kHz to 1 GHz. This paper proves usability of the concept by using two 18650 Lithium-ion cells connected in series.