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Reference measuring set-up for the state of health of Li ion battery cells

Owners of electric cars are basically interested in two things when it comes to the battery of their vehicle: the state of charge of the battery and its state of health. This means in the first place, how far can they get before they have to recharge their battery; and, in the second place, what is the remaining capacity of the battery, which, in contrast to fuel tanks, decreases over time. Electrochemical impedance spectroscopy is a promising measuring method to determine the state of charge and the state of health of lithium-ion battery cells. It is used to measure the AC resistance (impedance) at different frequencies. This method could be used for quality assurance purposes in the automobile industry, in workshops and directly in the vehicle. It also has a great potential in battery research in order to assess improvements in battery design or in the production process. In comparison to other methods, it has the great advantage that it is easy to handle and that it is purely electrical, cost-effective and fast. Above all, it provides an insight into the internal processes of a battery cell and how these processes change – without having to destroy the battery cell itself.

The benefit of electrochemical impedance spectroscopy is, unfortunately, often questioned, particularly in connection with the measurement of battery cells. As this method reacts very sensitively to changes in the measurement conditions and to the condition of a battery cell (e.g. temperature or degradation of the battery), it is often deemed ambiguous and difficult to replicate. This is why it has, so far, not been able to actually establish itself as a method to measure the condition of Li ion high-energy cells, even though, paradoxically, it is already widely used in practice.

We have established several electrochemical impedance measurement set-ups and were able to demonstrate in broad-based measurement series that impedances of Li ion high-energy cells can currently be measured with a measurement uncertainty of 1 to 3 %, provided that precisely defined measurement conditions are applied. We are presently also investigating the second point of criticism – the alleged ambiguity of the results. Based on the results of two life cycle test series conducted on several battery cells we could demonstrate that even at operation under different currents and temperatures, it is possible to determine the state of health unambiguously from the impedances measured.

To be able to ensure the reliability of the measurements, we are currently working on two further projects. On the one hand, the impedances measured must be reliably traceable to the International System of Units.  For this purpose, a reference spectrometer optimized for the frequency spectrum and for the impedance spectrum of Li ion high-energy cells is currently being developed. This spectrometer is intended for the calibration of reference impedances that will later serve as calibrators for electrochemical impedance spectrometers. On the other hand, the analysis of the life cycle tests has, to date, been based on a merely empiric methodology. To additionally underpin the results by electrochemical processes, the aging behavior of the impedance spectra is simulated by model calculations.

 
Opens internal link in current windowLink to the reference spectrometer

Opens internal link in current windowLink to the model calculations