Non-Zero Intercept Frequency: An Accurate Method to Determine the Integral Temperature of Li-Ion Batteries

A new impedance-based approach is introduced in which the integral battery temperature is related to other frequencies than the recently developed zero-intercept frequency (ZIF). The advantage of the proposed non-ZIF (NZIF) method is that measurement interferences, resulting from the current flowing through the battery (pack), can be avoided at these frequencies. This gives higher signal-to-noise ratios (SNRs) and, consequently, more accurate temperature measurements. A theoretical analysis, using an equivalent circuit model of a Li-ion battery, shows that NZIFs are temperature dependent in a way similar to the ZIF and can therefore also be used as a battery temperature indicator. To validate the proposed method, impedance measurements have been performed with individual LiFePO4 batteries and with large LiFePO4 battery packs tested in a full electric vehicle under driving conditions. The measurement results show that the NZIF is clearly dependent on the integral battery temperature and reveals a similar behavior to that of the ZIF method. This makes it possible to optimally adjust the NZIF method to frequencies with the highest SNR.