Data Sheets
BT2191A / BT2192A Self-Discharge Measurement System and Software
A New Way of Looking at Li-Ion Cell Self-Discharge
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:
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.
Self-Discharge Measurement System Software
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