In a previous blog, I wrote about how the AC method can be applied to measure lead acid batteries and how the principle of this measurement works. I also mentioned at the end of that blog post, that for Lithium-Ion (Li-Ion) batteries the measurement gets a bit more tricky. The reason for this is an additional impedance element in Li-Ion batteries, called the Warburg impedance. On the one hand, the size of the Warburg impedance, caused by the difusion of lithium in the electrode material, is a parameter of interest to characterize a Li-Ion battery. On the other hand, the presence of the Warburg impedance requires a slightly different measurement approach.
In this blog post, I will describe what the Warburg impedance is, how it affects the Cole-Cole plots and finally the different measurement approach.
Warburg impedance in the Li-Ion battery model
The lithium ions in the electrolyte move into the electrode material. There they take one of the free electrons and become lithium atoms. These lithium atoms then diffuse within the electrode material. The diffusion process can be modeled with the Warburg impedance, which adds an additional element to the equivalent circuit of the electrical double layer. Normally this only consist of a resistor in parallel with a capacitance. The presence and effect of the Warburg impedance is shown in the following schematic.
Warburg impedance in the electrode of a Li-Ion battery and the resulting equivalent circuit for modelling the battery.
Obviously, such an additional impedance element in the equivalent circuit for the electrical double layer between electrode and electrolyte, influences the overall equivalent circuit for a Li-Ion battery and hence the Cole-Cole plot. In particular, as depicted in the following picture, the Warburg impedance adds another portion of interest to the Cole-Cole plot. It is the part below 1 Hz and reaches down to roughly 10 mHz.
Exemplary Cole-Cole plot for a Li-Ion battery with the portion between 1 Hz and 10 mHz caused by the Warburg impedance.
Adapting the measurement method
Since the Warburg impedance is a parameter of interest in characterizing a Li-Ion batteries, the approach with a resistance measurement taken at 1 kHz, as implemented in the AC method for lead acid batteries, is not sufficient anymore. To obtain the impedance of the Warburg element and to be able to cover the part in the Cole-Cole plot below 1 Hz, the AC method needs to be extended from a single frequency point to a frequency range between 1 kHz and roughly 0.1 Hz.
This approach is taken in the Hioki BT4560 Battery Tester and provides the user information about the resistance of the electrolyte as well as on the Warburg impedance. As for the 3554 Battery Tester for lead acid batteries, the BT4560 eliminates the need to perform time consuming charge-discharge cycles.
If you need to evaluate Li-Ion batteries for example for your IoT, metering or wearable application, the Hioki BT4560 Battery Tester is the ideal tool helping you to achieve your results quickly and with just one single box.
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