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Keysight Solutions for Measuring Self-Discharge of Lithium Ion Cells Achieves Revolutionary Reduction in Test Time

2019-10-08  |  5 min read 

Ideally lithium Ion (Li-Ion) cells would be able to hold their electrical charge without any loss for all time. However, in the real world all Li-Ion cells have some amount of self-discharge. Self-discharge is the loss of charge over time while the cell sits unused and is not connected to anything. Some self-discharge is a normal attribute of cells. It is typically just tenths of a percent per month for Li-Ion cells. However, excessive self-discharge is not normal. It can be an indicator of latent defects. Hence cells with excessive self-discharge must be screened out from the main population of cells during manufacturing.

The traditional method of measuring a cell’s self-discharge is to measure the cell’s loss of open circuit voltage (OCV) over time. The OCV is an indicator of the cell’s % state of charge (SoC). Therefore, loss of OCV over time is an indicator of self-discharge. The major disadvantage of the OCV method is the time it takes; it takes typically weeks to get a meaningfully large enough drop in OCV such that it can be reliably measured. For manufacturing this means much greater work-in-process, with associated carrying costs, factory space, and associated risks with storing large quantities of Li-Ion cells.

An alternative approach to the OCV method is to instead use a potentiostatic method to directly measure the cell’s internal self-discharge current. Basically, a very high-performance DC source is set to precisely match the cell’s OCV and then connected to the cell through a microammeter. Ultimately the DC source holds the cell at a constant SoC, externally furnishing all the cell’s self-discharge current. This method takes typically hours or less for the measurement to settle out, and even less time to discern cells having excess self-discharge from good ones. An example of a potentiostatic measurement on several cells is shown in Figure 1

Figure 1: Self-discharge current measurements of Li-Ion cells using the Keysight BT2152B

To better understand how the OCV and potentiostatic methods for measuring self-discharge on Li-Ion cells work, Keysight has an application note that goes into the details of how each of these two methods work and the measurement science behind each. If this is of interest to you, click on the application note’s title; “Evaluate Self-Discharge of Lithium Ion Cells in a Fraction of the Time Traditionally Required” to learn more!

The newly-introduced Keysight BT2152B Self-Discharge Analyzer, as well as its successor, the BT2152A, use the potentiostatic method to dramatically reduce the time it takes to evaluate self-discharge of Li-Ion cells. In manufacturing this can greatly reduce work-in-process with its associated costs. Also, the BT2152B, together with the companion BT2155A Self-Discharge Analysis software, creates a useful solution for quickly evaluating self-discharge of cells in R&D, dramatically reducing development cycle time of new designs. The BT2152B and BT2155A are shown in Figure 2.

Figure 2: The BT2152B Self-Discharge Analyzer and BT2155A Self-Discharge Analysis software

Since introducing the BT2152A, there were several things learned from working with customers on how performance could be enhanced to further improve on discerning good versus bad cell in less time, that were incorporated into the BT2152B, including:

  • In situ wiring resistance calibration improves consistency across channel measurements.
  • Incorporated denoising algorithms that removes noise common across measurement channels.
  • More robust channel connectors for lower, more consistent contact resistance.
  • Improved measurement accuracy.
  • Greater stability and lowered electrical noise.

These things make the BT2152B Self-Discharge Analyzer’s provide even greater value in reducing costs for cell manufacturers, reducing the time of days to weeks using traditional methods of measuring open circuit voltage loss, to as little as minutes by directly measuring the self-discharge current.

To learn more about making cell self-discharge measurements and the BT152B Self-Discharge Analyzer by accessing the Keysight BT2152B product page at the following link: “BT2152B”.

Check back for future posts on self-discharge of lithium ion cells and how the BT2152B enhancements further optimize its measurement!