BT2152B Self-Discharge Analyzer

• Up to 32 channels of self-discharge current measurement available in 4-channel increments
• Cell voltage range: 0.5 to +4.5 V
• Current measurement accuracy: ± (0.30% + 250 nA)
• Voltage measurement accuracy: ± (0.04% + 0.1 mV)
• Voltage sourcing stability: ± 3 μV peak (24 hour), ± 0.85 μV peak (1 minute)
• Current measurement range: ± 10 mA

The Keysight BT2152B Self-Discharge Analyzer directly measures self-discharge current of Li-Ion cells. The analyzer provides a revolutionary reduction in the time required to discern good vs. bad cell self-discharge performance. This drives dramatic reductions in Work-in-Process inventory, working capital costs, and facility costs for cell manufacturers.

The Self-Discharge Analyzer accurately measures cell self-discharge current and cell voltage. Depending on the characteristics of the cells, measurements are directly made in minutes or hours instead of taking days or weeks using open-circuit cell voltage measurements to indicate which cells are good vs. bad.

The Self-Discharge Analyzer quickly measures self-discharge current using a precision potentiostatic measurement method with the characteristics needed to accurately make the current measurement:

• Minimum disturbance of the cell
  • The voltage applied to the cell is quickly matched to the actual cell voltage. This minimizes any new charge or discharge and thus limits any new RC settling to a minimum.
  • The voltage applied to the cell is very stable (± 3 μVpk) to minimize continuing charge-discharge current noise on the self-discharge current measurement.
• Accurately measures low-level self-discharge currents to ±(0.30% + 250 nA)

Process Improvements and Cost Savings in Self-Discharge Testing

Li-Ion cell manufacturers keep far greater numbers of cells in work-in-process inventory than they would like because of the time they are required to keep their cells in the aging process.

Today, most of the total aging time is often caused by the time required to determine if the cells’ self-discharge behavior is within acceptable limits. This large time period is driven by how long it takes for the change-in-OCV (ΔOCV) measurement. Reducing the amount of time cells spend in the aging step as work-in-process inventory provides savings that flow directly to the bottom line.

Improvements in the aging process and resulting cost savings from directly measuring self-discharge current can be seen by examining a typical model of the formation and aging process in cell manufacturing, and comparing the traditional OCV method vs. directly measuring self-discharge current of the cells.

Traditional ΔOCV Method

Determining good vs. bad self-discharge performance is based on a 2-step process. In the first step, the cells which are clearly good and clearly bad are separated from “suspect” cells by a ΔOCV test done after the 5-day aging period, where ΔOCV = (OCV2 – OCV1). The cells designated as “suspect” are then subjected to a longer aging period followed by a second ΔOCV test, where ΔOCV = (OCV3 – OCV2). Most of the cells require less aging with this process, but the “suspect” portion of the cells requires much longer aging to determine whether they exhibit acceptable self-discharge behavior.

Direct self-discharge current measurement

Determining good vs. bad self-discharge performance is again based on a 2-step process. The first step uses the same ΔOCV test where ΔOCV = OCV2 – OCV1. The “suspect” cells then have a direct self-discharge measurement. This measurement typically takes about 1 hour or less, eliminating the very long traditional “suspect” aging period, typically lasting 4 weeks or more.
If 10% of total cell production is classified as “suspect”, requiring additional test or aging beyond the first ΔOCV test, using direct self-discharge measurement reduces the total aging time for “suspect” cells by ~81% (7 vs. 37 days). This method provides a 30% reduction in the total number of cells in aging due to elimination of the long “suspect” aging step. This directly impacts Work-in-Process inventory and facilities requirements.

You can download a cost savings model for Working Capital Costs and Facilities Costs comparing the ΔOCV method vs. direct self-discharge measurement (SDM). This model is built using Microsoft Excel, and the model worksheet is available below for you to download and modify to fit your situation for each type of cell you make.