Impedance spectroscopy on lithium ion cells for automotive applications with optimized measurement duration and frequency resolution
Peter Haußmann, Joachim Melbert
18. GMA/ITG-Fachtagung Sensoren und Messsysteme, Nürnberg, AMA Service GmbH, 2016
A new impedance spectroscopy measurement procedure for automotive battery cells is presented, which is based on waveform shaping. The method is optimized towards short measurement duration, high excitation energy and increased frequency resolution and overcomes the limitations of established methods. For a given spectral amplitude profile, the corresponding time domain waveform is derived from the inverse discrete Fourier transform. Applying an identical initial phase angle for each frequency component, the resulting signal exhibits a high peak-to-peak amplitude at relatively low total excitation energy. This limits the maximum allowed energy for linear excitation. Altering the phase angles randomly spreads the excitation energy across the complete measurement duration. Thereby, linearity is preserved at higher excitation energy. By including window functions in the synthesis concept, spectral leakage is reduced without changing the spectral signal amplitude in the frequency range of interest. A time domain waveform optimized for impedance spectroscopy on lithium ion cells is synthesized based on the proposed approach and evaluated on real automotive cells. The resulting impedance data show good concordance with established standard measurement procedures at significantly reduced measurement duration and charge throughput. Additionally, increased frequency resolution is achieved, enhancing the level of detail of the obtained impedance data. The method is used for improved localization of aging effects in the cells, without further stress of the cells by the measurement procedure.