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New Resistance Calibration Method Improves Matching of Li-Ion Cell Self-Discharge Current Measurement Response Times

2019-10-21  |  6 min read 

In my previous posting “Keysight Solutions for Measuring Self-Discharge of Lithium Ion Cells Achieves Revolutionary Reduction in Test Time”, I introduced the two main methods of measuring self-discharge of lithium Ion cells, the traditional open circuit voltage (OCV) method and the potentiostatic method. I also introduced the Keysight solutions for measuring self-discharge, the BT2152B Self-Discharge Analyzer and BT2155A Self-Discharge Analysis software. The BT2152B uses the potentiostatic method to directly measure the cell’s self-discharge current. This method can achieve a revolutionary reduction in test time over the more traditional method of measuring self-discharge based on measuring the loss of the cell’s OCV over days to weeks of time.

Drastically reducing the time required to measure self-discharge of cells does have some implications. The conditions need to be much more carefully controlled and matched across the lot of cells being tested. This includes things like:

  • The temperature of the cells is consistent and kept stable.
  • The cells are all at the same state of charge (% SoC).
  • The series resistance value between the cells and corresponding potentiostatic sources are accurately set and well-matched across all the cells.

For the third item listed above, the series resistance value, in combination with the cell’s capacity, determines the response time of the self-discharge current measurement. This can be understood from Figure 1. At time zero, the potentiostatic source is set to match the cell’s open circuit voltage and the switch is closed. The initial current flow is zero. At equilibrium, the current externally provided by the potentiostatic source equals the cell’s self-discharge current. The cell is being held at a constant % SoC when at equilibrium. The time to reach equilibrium is determined by the time constant formed by the series resistance and effective capacitance (i.e. capacity) of the cell.

Figure 1: Potentiostatic method of self-discharge current measurement

The BT2152B features programmable series output resistance setting as a means of optimizing measurement response time. However, the value of the external wiring resistance needs to be very accurately known or determined in order to end up with an accurate total series resistance setting. This can be understood from Figure 2. This becomes increasingly important as the total series resistance is set to lower values in order to reduce measurement settling time, as the error becomes a larger portion of the overall setting.

Figure 2: BT2152B wiring resistance and total series resistance setting

When the total series resistance setting is not well-matched across all the cells being tested, the measurement response times are correspondingly poorly matched, making discerning good cells from cells having high self-discharge problematic.  This is illustrated in Figure 3, where a set of cells were tested for self-discharge, with poorly-matched series resistance settings.

Figure 3:  Cell self-discharge current measurements for poorly-matched total series resistance

 

Many factors affect the wiring resistance value which leads to a poorly-matched total series resistance setting across the set of cells being tested. One very prominent factor is variability of contact resistance to the cells' terminals. This can greatly vary each time another set of cells is connected up. This in turn leads to poorly-matched measurement response times across the set of cells being tested. To address this, Keysight has incorporated an In-situ wiring resistance calibration system into the BT2152B. This system accurately measures the test setup wiring resistance with the cells already in place, avoiding any variability introduced by the contacts each time another set of cells are connected to be tested.

When the total series resistance setting is well matched across all cells being tested, the measurement response times are correspondingly well matched. This can be seen in Figure 4, where the same set of cells used for Figure 3 were tested with well-matched total series resistance settings.

Figure 4: Cell self-discharge current measurements for well-matched total series resistance

When the total series resistance is well-matched across all the cells being tested, the cells’ self-discharge current measurements maintain their relative positions and rankings throughout the measurement, not just for the settled values at the end. This makes for much better determination of discerning good cells from cells having high self-discharge and in less time.

If self-discharge of lithium ion cells is important to you, click on: “Matching Response Times of Lithium-Ion Cell Self-Discharge Current Measurements” to open and download this application note from Keysight. Learn more about the importance of the series resistance setting when measuring self-discharge currents on lithium ion cells, how the BT2152B in-situ wiring resistance calibration system works, and more, as the application note goes into much greater detail on these topics.