Abstract
The power capability of a lithium ion battery is governed by its resistance, which changes with battery state such as temperature, state of charge, and state of health. Characterizing resistance, therefore, is integral in defining battery operational boundaries, estimating its performance and tracking its state of health. There are many techniques that have been employed for estimating the resistance of a battery, these include: using DC pulse current signals such as pulse power tests or Hybrid Pulse Power Characterization (HPPC) tests; using AC current signals, i.e., electrochemical impedance spectroscopy (EIS) and using pulse-multisine measurements. From existing literature, these techniques are perceived to yield differing values of resistance. In this work, we apply these techniques to 20 Ah LiFePO4/C6 pouch cells and use the results to compare the techniques. The results indicate that the computed resistance is strongly dependent on the timescales of the technique employed and that when timescales match, the resistances derived via different techniques align. Furthermore, given that EIS is a perturbative characterisation technique, employing a spectrum of perturbation frequencies, we show that the resistance estimated from any technique can be identified – to a high level of confidence – from EIS by matching their timescales.
Highlights
Batteries play a significant part in powering modern technology, from consumer goods to electric vehicles and renewable energy storage systems[1]
Measurement of the instantaneous drop is limited by the data acquisition rate of the equipment used
If a higher current like 1 C was used for the electrochemical impedance spectroscopy (EIS) test, it would have an effect on results, application of such high current for EIS test has a little precedent in literature
Summary
Batteries play a significant part in powering modern technology, from consumer goods to electric vehicles and renewable energy storage systems[1]. The performance and efficiency of a lithium ion battery is largely governed by the resistance of the electrochemical system. In addition to thermal gradients across the battery pack, thermal gradients can develop across individual cells, both along and normal to the electrode stack, due to inhomogeneous local current distributions under operational conditions, or internal manufacturing defects[5]. Such inhomogeneity results in localised heating, leading to local cell temperature ‘hot spots’ approaching values close to which the separator can melt leading to thermal runaway[6]. The value of resistance measured will depend on the remaining degree of freedom: the measurement duration (timescale) of the measurement, which is related to the underlying electrochemical process involved
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