Shortening Lithium Ion Cell Manufacturing Time: A Comparative Study of Two Methods of Making Self-Discharge Measurements
Last year at The Battery Show Conference in Novi Michigan, I presented this topic I prepared; “Shortening Lithium Ion Cell Manufacturing Time: A Comparative Study of Two Methods of Making Self-Discharge Measurements”. I am now sharing it here as I think many should find it of interest. Self-discharge is a critical parameter that all lithium ion cells get screened for in manufacturing, as excessive self-discharge is indictive of underlying defects within the cell. There are two primary methods for its measurement.
The first method is the traditional delta OCV method, where loss of the cell’s open circuit voltage (OCV) is measured over typically over weeks of time. The amount of loss in OCV is an indirect indicator of the amount self-discharge in the cell. The second method is the potentiostatic method. This involves holding the cell at a constant state of charge (SoC) by holding the cell at a fixed potential with a stable external source. After settling on the order of an hour or so to equalize, the current being furnished by the external source equals the cell’s self-discharge current. I have previously written about these two measurement methods in much greater detail. For reference, “Keysight Solutions for Measuring Self-Discharge of Lithium Ion Cells Achieves Revolutionary Reduction in Test Time” (click on title to access) is a worthwhile post to review to learn more about cell self-discharge and its test methods. Let’s now look to see how the two methods compare.
As indicated, shortening manufacturing test time is paramount. What limits the test time for measuring self-discharge in cells, using the traditional delta OCV method? This can be understood by referring to Figure 1.
Figure 1: What limits the delta OCV method test time
The delta OCV method relies on measuring a very small voltage drop that is on top of a very large DC offset voltage; the cell’s OCV. The decay rate is governed by the parallel combination of the cell’s effective capacitance, CEFF and internal self-discharge resistance, RSD, as shown on the schematic on the right. CEFF is on the order of 10,00 farads and RSD is on the order of 10’s of kilo-ohms for a 2Ah 18650 cell, making the OCV decay rate very slow. For a group of 8 cells tested for this topic, the main population had a self-discharge OCV drop under 1.1 mV over a 7-day period. In comparison, two of the cells exhibiting high self-discharge OCV loss a little over 2.2 mV over the same 7-day period, as shown on the graph on the left. 7 days was about the minimum amount of time needed in order to get a valid result with acceptable uncertainty and error due to factors, including:
- DVM 10-volt measurement range accuracy and temperature coefficient
- Thermal EMF errors generated by electrical contacts
- Cell OCV temperature coefficient
Up to two weeks is often needed for a valid result using the traditional delta OCV method, depending on how well these errors can be controlled. The advantage of the longer rest time is that there is a greater voltage drop result, proportionally reducing the magnitude of the sources of error. The main downside of the delta OCV method is that the cells lay dormant in storage during weeks of time, causing a lot of work-in-process (WIP), more factory space for storage, and the associated hazards of having to store large volumes of cells.
In comparison to the OCV method, what shortens the test time for measuring self-discharge on cells, using the potentiostatic method? This can be more easily seen by referring to figure 2.
Figure 2: What shortens the potentiostatic method test time
The potentiostatic method directly measures the cell’s self-discharge current, with no offset to contend with. Equipment measurement accuracy and temperature coefficient are no longer a concern as a result. Instead of being open-circuited, the cell is basically short-circuited by the external potentiostatic source, which was first set to match the cell’s OCV, before being connected. The measurement settling time is now determined by the parallel combination of CEFF and the potentiostatic setup’s settable RSERIES resistance, as shown in the schematic on the right. In practice RSERIES is in the range of an ohm or less. This is orders of magnitude smaller than RSD, making the potentiostatic method response time orders of magnitude faster than the delta OCV method. As shown in the graph on the left, the measurement settling time is on the order of an hour but discerning the cells with high self-discharge can typically be determined well before that. The actual potentiostatic method measurement is shown in Figure 3. This was performed using a Keysight BT2152B Self-Discharge Analyzer and BT2155A software, on the same group of 8 cells that the delta OCV measurement had previously been performed on. For reference, you can learn more about the BT2152B and BT2155A on the Keysight page “Self-Discharge Measurement Solutions” (click on page name to access).
Figure 3: Potentiostatic method test results
There are tradeoffs with the potentiostatic method. The main consequence of reducing test time that needs to be contend with is greater sensitivity to the temperature coefficient of the cell’s voltage. The cell’s temperature needs to be very well controlled or its effect can exceed that of the self-discharge current itself. Due to the relatively short test period the cell’s temperature changes can be kept to a minimum typically by using simple passive methods, like thermal insulation.
Having performed both the delta OCV method and potentiostatic method on the same group of cells, how well do the two self-discharge measurement methods correlate? This is illustrated in Figure 4.
Figure 4: How results of the two methods correlate
The blue dots are the measurement points on each of the eight cells, with the delta OCV loss rate on the X-axis, in µV/hr, and with the self-discharge current on the Y-axis, in µA. It can quickly be seen that all the points linearly line up by the red line, and its extrapolation passes through the plot’s origin. These all indicate very strong correlation. Further, the ratio of the two methods’ results agrees with the discharge slope characteristics for these cells at the %SoC they were tested, providing additional validation there is good correlation between the two methods. To achieve these results good test practices were required, including:
- Cells were fully rested, eliminating effects from charge redistribution.
- The two methods were performed concurrently on the same group of cells.
- Environmental conditions were kept as consistent as possible.
- Temperature influences on the cells and equipment were minimized.
In closing, the two main methods of measuring self-discharge on lithium ion cells are the traditional delta OCV method and the potentiostatic method. Due to the very low rate of OCV loss, the delta OCV method takes up to weeks of time, greatly increasing WIP, require storage space, and present an additional hazard. The long time for the delta OCV method is used to provide a meaningful result in comparison in relation to the magnitude of measurement error sources present. In comparison the potentiostatic method has a measurement settling time on the order of an hour, providing opportunity to significantly reduce manufacturing time and WIP. The consequence of such a fast test time is a greater sensitivity to temperature changes, mainly due to the cells themselves. With proper steps taken to minimize temperature change during the testing, this can be mitigated. Either method will produce results that accurately quantify the self-discharge, and will correlate between each other, as shown here, if they are properly performed under comparable conditions.