How to Screen Lithium-Ion Cells for Defects

+ Battery Self-Discharge Analyzer

Screen Cells Using Self-Discharge Current

Lithium-ion cell screening based on self-discharge current requires measuring cells under comparable electrical, thermal, and state-of-charge conditions so that abnormal current behavior can be separated from the normal cell population. With the potentiostatic method, a low-noise, stable direct current source is first adjusted to closely match the open-circuit voltage (OCV) of each cell. The source is then connected to the cell through a low-current measurement path and holds the cell at essentially constant voltage and state of charge (SOC). Internal self-discharge continues to remove charge from the cell, but the external source replaces that charge by supplying current. As the source and cell approach equilibrium, the externally supplied current becomes the cell’s self-discharge current. For lithium-ion cell screening, engineers can compare this current across multiple cells and observe how individual current-versus-time traces relate to the main population. Cells exhibiting persistently higher self-discharge can separate from normally behaving cells before every channel reaches its final equilibrium value. Direct self-discharge current measurement provides a measurement of charge loss over time instead.

Measurement conditions must be controlled when applying this technique to battery defect detection, because cell and environmental behavior can produce current transients that are unrelated to the final self-discharge current. Charge redistribution following charging or discharging is one source of settling behavior. Temperature is another significant variable. Lithium-ion cell voltage can exhibit a temperature dependency of many tens of microvolts per degree Celsius, and changes in cell voltage can cause corresponding fluctuations in measured self-discharge current. State of charge also affects lithium-ion cell self-discharge because self-discharge decreases considerably at low SOC and becomes more difficult to measure, while the cell temperature coefficient of voltage generally increases. Programmable series resistance can be used to adjust the tradeoff between measurement settling time and sensitivity to external influences such as temperature changes. When these variables are controlled consistently across multiple channels, population-based battery manufacturing test data can be used to distinguish cells exhibiting excessive self-discharge from the main population and retain the associated current, voltage, and temperature histories for subsequent engineering analysis.

Lithium-Ion Cell Manufacturing Screening Solution

Perform the screening test by measuring self-discharge current on multiple lithium-ion cells under comparable potentiostatic conditions and evaluating each current trace relative to the overall cell population. The Keysight Self-Discharge Analyzer provides up to 32 simultaneous measurement channels, directly supporting parallel lithium-ion cell screening in manufacturing applications. A thermistor measurement port enables cell temperature to be tracked during the test, while programmable series resistance provides a means of optimizing settling time and sensitivity to external influences. Built-in logging continuously records cell voltage, current, and temperature after measurement begins. The Self-Discharge Analysis Software provides an interactive graphical interface for configuring the analyzer and evaluating the resulting multi-cell measurements. For battery manufacturing test applications, engineers can display multiple current traces simultaneously and identify cells that separate from the main self-discharge population.

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