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BT2191A / BT2192A Self-Discharge Measurement System and Software

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BT2191A / BT2192A Self-Discharge Measurement System and Software

A New Way of Looking at Li-Ion Cell Self-Discharge

  • Characterize self-discharge current in minutes or hours instead of days or weeks
  • Faster evaluation results, faster design iterations
  • Software for control, graphing, logging, data storage
  • Eliminates days or weeks of cell storage time

The Challenge in Evaluating Self-Discharge

It’s a challenge for Li-Ion cell designers to quickly measure the self-discharge behavior of their cell designs. And it’s equally challenging for the users of Li-Ion cells to evaluate the self[1]discharge behavior of the cells they’re considering for use in their electronic equipment and battery pack designs.

The challenge isn’t that it’s a complicated measurement – the challenge is that it’s a very time-consuming measurement. Today, the measurement is typically done by measuring how much the open-circuit voltage (OCV) of the cell changes over time, as an indicator of how much the state-of-charge (SoC) changes due to self-discharge. And since most Li-Ion cells have very little change in OCV as the cells discharge, it takes a long time to see changes in the (SoC) of the cells. This process can take weeks or months, depending on the cell.

While the time you spend on any one cell measurement isn’t very long, the fact that there’s a series of these measurements spread over time has a big impact on your design cycle time. The time from starting the self-discharge evaluation to its conclusion can take weeks or months, even if you aren’t spending all your time on cell evaluation during that time. And that makes a real impact on your time to market - either as a cell designer working on a new cell, or a cell evaluator working on the design of equipment that will use the cell you’re evaluating. When you work on a cell design, you charge the cells, allow charge redistribution to finish, and then start the self-discharge evaluation process. You measure the OCVs, then put the cells into storage while you wait for the OCV to change. You likely must store the cells under temperature-controlled conditions since the cell voltage varies with temperature. And then you turn your attention to other designs or tasks.

When you come back to those cells, there’s always a bit of a learning curve to reacquaint yourself with something you’ve already started. It’s just not as efficient as it would be if you could see the self[1]discharge picture without waiting. And this problem is worse for larger capacity cells, which is where a lot of the market growth is these days. Large cells inherently have a more complex test setup and storage issues due to required safety precautions.

The Real Impact of the Challenge

If the time to characterize self-discharge is a gating task in your cell design or evaluation cycle, the number of extra weeks it takes to complete the self-discharge measurement is essentially the number of extra weeks it takes to either get your cell design to market or to get your equipment design to market. And if you need multiple test cycles as you iterate your design, then the delay is multiplied by the number of test cycles you go through. All of which becomes opportunity loss because you didn’t get your design to market before its competition.

A Better Way to Evaluate Li-Ion Cell Self-Discharge

To measure the self-discharge performance of a cell, you would like to directly measure the selfdischarge current of the cell. A potentiostatic measurement system capable of making this current measurement must have these important characteristics:

  • The measurement equipment connected to the cell must create minimum disturbance to the state-of-charge (SoC) of the cell.
  • The voltage applied to the cell by the test equipment must be held equal to the cell voltage.

Otherwise, the cell will either charge or discharge, and you will initiate charge redistribution currents as well as RC settling currents that will mask the self-discharge current you’re trying to measure.

  • The voltage applied to the cell must be very stable. Otherwise, any instability or noise in the applied voltage will cause charge redistribution currents that will show up as noise on the self[1]discharge current measurement.
  • The test equipment must accurately measure low-level self-discharge currents in the range of 10’s of μA.

Self-Discharge Measurement System Software

  • Measures and records cell self-discharge current, cell voltage, cell temperature
  • Configures the instruments in the system.
  • Saves or logs measurement data.
  • Recalls previously stored measurements for display and analysis.
  • Exports recorded data to Microsoft Excel (xlsx file)
  • Matching function measures initial cell voltage and adjusts applied voltage to match so an accurate self-discharge measurement is obtained more quickly.
  • Allows the user to adjust the effective total resistance value (including the physical resistor) in series with the cell. This allows the user to select a total resistance value to optimize the RC settling time of the measurement. A lower resistance gives faster settling time but increases current measurement noise due to voltage fluctuation with temperature.

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