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-zero-intercept frequency
(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 (SNR) 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.