A Novel Method for High Frequency Battery Impedance Measurements

Electrochemical Impedance Spectroscopy (EIS) is widely used to measure the impedance of lithium-ion (Li-ion) battery cells. The EIS focuses on frequencies from millihertz to kilohertz, since the electrochemical processes do not have shorter time constants. To investigate high frequency phenomena as the electromagnetic compatibility (EMC) of an automotive traction battery, impedance measurements also in the higher megahertz range are necessary. State-of-the-art EIS measurement devices for batteries do not meet the requirements of this application, as they can cause electromagnetic wave reflections and do not provide sufficient calibration techniques. In this paper, we present a method to determine the battery impedance for a wide frequency range from 1 kHz to 300 MHz. Using a vector network analyzer, two-port scattering parameters (S-parameters) of a 18650 Li-ion cylindrical cell are measured with the shunt-through method. The resulting cell impedance is 40 Ωm at 1 kHz and increases to 40 Ω at 300 MHz mainly due to the external inductance of the cell.