In this article we will discuss how impedance measurement, a widely used battery test technique, is implemented in the ModenJay Platform.
(Figure 1) Recipe Icons for AC Resistance and Frequency Response
Measuring the impedance across a cell is a very popular method of measuring the cell’s internal properties. The real part of the measured impedance represents the internal resistance of the cell and the imaginary part represents the capacitance due to irregular charge distribution within the cell.
The ACR recipe measures the impedance by measuring the voltage output with respect to the sinusoidal current input. The user can control the property of the sinusoidal current by setting the frequency, amplitude and the bias of the input signal. Depending on the size of the output signal, the user can select the precision of voltage scope for enhanced precision of the impedance measurement. The ACR measurement step is automatically terminated once the impedance value is returned.
The FR recipe measures the frequency response of the cell, ie) the impedance spectrum with varying frequency of the input signal. The essential concept of FR is to repeat ACR measurement in a customized range of frequencies and retrieve the measured data. The user can also control the properties of the signal, along with the frequency range and the number of frequency values to sweep for FR. FR is also widely known as impedance spectroscopy. The measurement is terminated once the spectrum is obtained.
(Figure 2) FR Recipe measurement properties
Metadata for the ACR and FR recipes consist of a common part and a unique part. The common part records the experiment background information including the sample information and the test log, which were elaborated in the previous article “Test Recipe (3) Metadata”.
The unique part is where the raw result data for the ACR and FR recipes are converted into metadata files. A group of chosen raw data are directly extracted to be a part of the ‘Extracted Data’ section of metadata. These include the real and imaginary parts of the measured impedance values at certain frequencies. The ‘Characteristic Parameters’ aim to fully represent the result by recording the impedance values at the predetermined characteristic points, and the parametric resistance values.
The characteristic points for the FR recipe can be predetermined in arbitrary ways. An example plot of McScience’s default setting is given below. It follows a widely accepted theory that the impedance spectrum of a lithium-ion cell is composed of three distinguished parts – the series resistance, the charge-transfer resistance and the diffusion resistance. After an FR measurement, the parameters are automatically calculated and recorded.
(Figure 3) McScience’s default FR characteristic points and parametric resistance values
(Figure 4) An example of the Extracted Data section for the FR recipe
(Figure 5) An example of the Characteristic Parameter section for the FR recipe
There are two EigenPlots for the FR recipe. One plots the imaginary part against the real part of the impedance values, known as the Nyquist plot. The other plots the real and imaginary parts of the impedance against the frequency of the input signal, known as the Bode plot. The two plots are widely accepted to represent the FR data very well.
(Figure 6) Examples of the EigenPlots for the FR recipe
Thank you for reading this article and we will discuss some other battery test recipes in the next article.
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