Here we developed calibrated and accurate impedance spectroscopy characterization of Lithium-Ion cells and modules with micro-Ohm resolution. Currently, the accuracy of battery impedance spectroscopy strongly depends on the mechanical test fixtures where a four-terminal connection is brought as close as possible to the battery connectors. In combination with a Short compensation step this arrangement is typically valid only up to certain frequencies (~ 1kHz) and impedance levels (> 1m Ohm), with higher frequencies or lower impedance levels leading to large systematic errors, allowing only for interpretation of relative impedance changes.
While modern test equipment easily allows to address single digit µOhm detection levels by using sufficient excitation currents, it is much less straightforward to maintain the accuracy of the measurement at these low impedance levels. We show here quantitative impedance spectroscopy that is based on a reliable calibration method which enables to compare results from different measurement setups. Also, we demonstrate how controlling fixture stability and drift in combination with an appropriate error model allows to significantly improve the accuracy at higher frequencies, up to 1 MHz, particularly in the low-impedance regime of 100-500 µOhm. This improved repeatability at low impedance levels is particularly relevant for modern prismatic Lithium-Ion cells and modules used in the automotive industry where we show some example measurements. The increased reproducibility of low impedance measurements is important for high volume validation in production lines, for the evaluation of the SOH, and for 2nd life applications, as exemplified in this presentation.
Nawfal Al-Zubaidi R-Smith, Keysight Technologies
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