In this article we will discuss testing a cell by measuring the internal resistance and using the technology called transient response analysis as in the ModenJay Platform.
(Figure 1) Recipe icons for DC Resistance and Transient Response
Measuring the DC internal resistance is the one of the simplest methods of testing a secondary cell. A DC current step is applied to measure the change in the cell’s voltage to calculate the DC resistance of the cell.
The DCR recipe measures the DC internal resistance as described above. The IEC61960-2003 international standard defines the DC resistance as the change in voltage over the change in current when a discharge current of 0.2C is applied for 10s followed by a discharge current of 1C for 1s.
The above equation describes this, where V1, I1 are the voltage and current values after the 10s 0.2C discharge just before the current step and V2, I2 are the voltage and current values after the 1s 1C discharge.
In ModenJay Battery Laboratory the IEC standard is applied as the default setting for the DCR recipe, but the user can always customize their own current step profile.
Transient response analysis is a rather new method of testing the performance status of a cell. Based on the time-frequency conjugacy in physics, it aims to capture all the information represented in the frequency response test (or impedance spectroscopy) and possibly more. The basic functionality is equal to measuring the DC resistance. However in the TR recipe as the current step is applied the change in voltage is monitored with the time resolution being as precise as in the microseconds, revealing the high-speed dynamics occurring within a cell. By time-frequency conjugacy one can argue that the microsecond-level dynamics is comparable to the megahertz-level results of impedance spectroscopy.
At McScience, a thorough research about the link between the FR and the TR results is in progress. So far there are evidences of similarity of the two methods. If TR becomes well known enough to be able to replace FR, one of its advantages is the fact that it is much less time-consuming. An ongoing current step captures all the dynamics naturally, but the sinusoidal signal can only be applied in discrete frequency steps. This means that the TR measurement is not only less time-consuming but also it could reveal some of the dynamics that are difficult to observe in the FR measurements.
Another example of TR being a more complex method is that at a given SOC, a current step can be applied in a few different ways, namely discharge and charge currents as well as a ‘turning on’ step and a ‘turning off’ step. A combination of these produces four different types of TR results whereas the FR can only produce one result at a given SOC. The different types of TR results may be able to reveal previously never known dynamics within a cell.
(Figure 2) TR recipe measurement properties
Metadata for the DCR and TR 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 DCR and TR 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 measured dynamic resistance values at certain times. The ‘Characteristic Parameters’ aim to fully represent the result by recording the resistance values at the predetermined characteristic points, and the parametric resistance values.
The characteristic points for the TR recipe can be predetermined in arbitrary ways. An example plot 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 a TR measurement, the parameters can be automatically calculated and recorded.
(Figure 3) An example of the TR characteristic points and parametric resistance values
(Figure 4) An example of the Extracted Data section for the TR recipe
(Figure 5) An example of the Characteristic Parameter section for the TR recipe
There are two EigenPlots for the TR recipe. One plots the voltage against the time of measurement and the other plots the dynamic resistance value against the time of measurement. The two plots are believed to represent the TR result very well.
(Figure 6) Examples of the EigenPlots for the TR recipe. In the second plot the time axis is in logarithmic scale to show the high-speed dynamics in the microseconds.
Thank you for reading this article and we will discuss some other battery test recipes in the next article.
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