Dmitri L. Danilov

Battery aging and its influence on the electromotive force

Li-ion is currently the most commonly used battery chemistry in portable applications. Accurate state-of-charge (SOC) and remaining run-time indication for portable devices is important for user convenience and to prolong the lifetime of batteries. The actual SOC algorithms, which the main companies use in practice, make use of the so-called electromotive force (emf). In these SOC systems it is assumed that the emf of an Li-ion battery only depends on aging to a limited extent. In this paper, novel emf measurement and modeling efforts are presented as a function of battery aging. As will be shown, a better understanding of this dependence is useful for improving SOC accuracy.

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.