Pick the Right Li-Ion Cell for Your Applications

Introduction

Most Li-Ion cells will gradually discharge even if they're not connectd to anything. This loss of stored energy leads to lower-than-desired available capacity from the cell. When cells are assembled into multiple-cell battery packs, differing rates of cell self-discharge lead to cell imbalances within the battery. Typical battery management systems will discharge all the cells to the level of the lowest cell, decreasing effective battery life

Use Case Summary

It’s a challenge for Li-Ion cell manufacturers to quickly discern whether newly formed cells exhibit acceptable self-discharge behavior. The open circuit voltage (OCV) method relies on measuring a very small loss of cell voltage over a long period of time to serve as an indirect indicator of the cell’s self-discharge rate. It traditionally takes days to weeks. Ideally, design and test engineers should measure the self-discharge current of the cell to speed up the process of identifying whether a cell is good or bad.

The self-discharge current flows through the parallel resistance RSD , as shown in Figure 1, which acts to decrease the voltage on the effective capacitance of the cell. But the potentiostatic analyzer holds the cell voltage constant because the analyzer supplies current to the cell equal to the cell’s selfdischarge current. The analyzer accurately measures the supplied current, quickly making necessary direct current (DC) measurements with minimum disturbance to the cell-under-test.

Solution Overview

Keysight’s Self-Discharge Measurement Solution controls voltage changes, eliminating significant self-discharge current measurement error. It quickly measures self-discharge current in minutes or hours, and includes integrated software to control the hardware, graph results, and log and store data. Measuring self-discharge current in hours saves the days to weeks normally required for opencircuit voltage to change enough to get an accurate self-discharge measurement.

Learn more at: Matching Response Times of Lithium-Ion Cell Self-Discharge Current Measurements

Summary

• OCV method takes too long.

• It is challenging to characterize a large number of cells using OCV method since it is slow, and can lead to safety and cost challenges.

• A highly accurate self-discharge measurement system can help to quickly characterize and grade cells under test.

For more information: Characterize Self-Discharge Current