The newly introduced Keysight SL1035A/SL1036A family of cell test systems rounds out the range of Keysight offerings at the low-current end, providing up to 96 5-amp channels or up to 48 10-amp channels in a single rack system. An example system is illustrated in Figure 1.
Figure 1: New Keysight SL1035A/SL1036A low-current cell test systems
While low-current cell test systems are useful for small lithium-ion cell development, they also play a vital role for developing large-format cells as well. Cell developers may be working on developing large-format lithium-ion cells capable of 100’s of amps, and a lot of the testing is directly conducted on them with high-current cell test systems. However, these test systems are limited in their number of channels due to the high current and power involved. Often, a lot of supplementary testing is also performed on smaller-scale versions of these large-format cells. They address a strategic need for tests on cell designs and chemistries that are benefited by using smaller-scale cells and having a greater number of cells. The new Keysight SL1035A/SL1036A low-current cell test systems address this strategic need in Keysight’s overall portfolio of cell test systems.
Tests run on a greater number of cells, even if they are smaller, provides several benefits, including:
- Greater insights on cell-to-cell variability.
- Higher levels of statistical confidence for life-cycling and other tests.
- More detailed information for data analysis.
- Greater efficiency by conducting tests on less runs or on adding variances within runs.
- And more.
The SL1035A test systems operate off 380~400 VAC 3 phase while the SL1036A test systems operate off 200~208 VAC 3 phase. There are three options for different channel offerings:
- Option 896: 96 channels, + 5A, 6V
- Option 860: 60 channels, + 5A, 6V
- Option 848: 48 channels, + 10A, 6V
Their high channel count addresses the need for testing larger numbers of cells, greatly benefiting life cycle and other types of testing relying on statistics. For some additional insights on life-cycle testing, please review my article by clicking on its title “Lithium-Ion Cell Charging and Discharging During Life Cycle Testing versus Formation”.
Certain measurements are more practical when using small-scale cells as well. This includes performing electrochemical impedance spectroscopy (EIS) and cyclic voltammetry measurements on half-cells when using a 3-electrode cell test setup.
A whole cell is a combination of two chemical reactions simultaneously taking place at both the anode and cathode sides of the cell. One is undergoing reduction (gaining electrons) while the other is undergoing oxidation (losing electrons). This is referred to as a redox reaction. For rechargeable (secondary) cells the reverse of the reaction takes place when recharging the cell.
In cell R&D it is not only important to evaluate the overall cell, but it is also important to evaluate the anode and cathode sides of the cell separately, to determine the relative performance of each half, i.e., half-cell testing.
Figure 2: Three-electrode cell test set up
Half-cell testing requires a special three-electrode cell assembly that introduces a third electrode which is the reference electrode. The reference electrode is placed between the anode and the cathode within the cell. Depending on whether the anode or cathode-half of the cell that is being evaluated, that electrode becomes the working electrode, and the other electrode becomes the counter electrode. Voltage is measured and regulated between the working and reference electrodes. A three-electrode measurement setup is schematically depicted in Figure 2. As can be seen, the test system must have additional suitable connections for supporting the reference electrode in addition to the working and counter electrodes. The output channels of the Keysight SL1035A/SL1036A cell test systems incorporate reference electrode inputs to support testing with three-electrode cell test set ups. By design, three-electrode cell assemblies are small-scale cells for practical purposes, thus better supported on a low-current cell test system like the Keysight SL1035A/SL1036A.
EIS measurements are a plot of the cell’s complex impedance over a wide range of frequency. Different frequency ranges correspond to different electrical and electro-chemical characteristics, providing insight into the cell’s performance and quality. A Nyquist plot of an EIS measurement is illustrated in Figure 3. Shifts in EIS plots can identify and quantify changes due to temperature or state of charge (SoC). Combined with life-cycle testing, shifts in EIS plots can identify different types of wear-out mechanisms as well. While EIS measurements are performed on a whole cell, they are also performed separately on each half-cell of a three-electrode test set up, to evaluate the relative contribution of each half. To learn more about impedance and EIS measurements on cells, click on the title of my article “A Deeper Look at Lithium-Ion Cell Internal Resistance Measurements”.
Figure 3: EIS measurement plot on a lithium-ion cell.
Another test performed on half-cells is cyclic voltammetry. This tests the reversibility of the chemical reaction of the half-cell. As the name implies, a cyclic sawtooth voltage is applied, charging and discharging the half-cell while its current response is measured. One thing looked for in a cyclic voltammetry plot is its symmetry. The more symmetrical the plot reflects the more reversible the charge/discharge reaction is. An example of a voltammogram plot is illustrated in Figure 4, having a characteristic symmetrical “duck” shape. Cyclic voltammetry tests can be made using Keysight SL1091A Energy Storage Discover (ESD) software together with the SL1035A/SL1036A cell test systems.
Figure 4: Voltammogram plot
In closing, the Keysight SL1035A/SL1036A cell test systems, together with the SL1091A Energy Storage Discover (ESD) software create a complete solution for testing smaller cells, rounding out Keysight’s portfolio of cell test systems. Interested in learning more about the Keysight SL1035A/SL1036A low-current cell test systems? If yes, then just click on the following links to access the home pages for “SL1035A 400VAC in low-current cell test systems” or “SL1035A 208VAC in low-current cell test systems”. Also, look here for future articles where I will go into more detail about the different types of testing, such as some of the test topics I have touched upon today!