How to Characterize Lithium-Ion Self-Discharge Versus Temperature

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Correlate Self-Discharge Current With Temperature

Characterizing temperature effects during battery self-discharge measurement requires simultaneous observation of cell current, voltage, and temperature because potentiostatic self-discharge measurements operate at electrical levels where relatively small thermal changes can influence the measured result. The test begins with a well-rested lithium-ion cell and a low-noise, highly stable direct current source adjusted to closely match the cell’s open-circuit voltage (OCV). The source is connected through a microammeter and maintains the cell at essentially constant voltage and state of charge (SOC). As internal self-discharge removes charge, the external source supplies replacement current. Once equilibrium is reached, the externally supplied current corresponds to the cell’s self-discharge current. Temperature must be monitored throughout this self-discharge current measurement because lithium-ion cell voltage exhibits temperature dependency. A stable thermal environment allows the electrical measurement to approach equilibrium with fewer temperature-induced disturbances. Continuous, time-aligned records of current, voltage, and temperature provide the data needed to evaluate whether current fluctuations coincide with thermal changes or whether the current is progressing toward a stable self-discharge level.

Thermoelectric effects must also be controlled during battery temperature testing because electrical connections between dissimilar metals can produce thermally induced voltages on the order of tens of microvolts per degree Celsius. Cell terminals may be made from nickel or another metal that creates such a voltage when connected to a dissimilar conductor. Connections at both cell terminals can provide opposing thermoelectric voltages that cancel when both terminals are at the same temperature, but a temperature difference between the terminals produces a net voltage. Changes in that temperature difference can therefore produce corresponding fluctuations in measured self-discharge current. State of charge introduces another variable in lithium-ion cell testing. Maintaining a reasonably high and consistent SOC therefore provides more suitable conditions for evaluating low-level current while controlling temperature-related voltage effects. Programmable series resistance can further adjust measurement settling time and sensitivity to external influences such as temperature changes. During acquisition, the cell should remain in a thermally stable environment, and temperature differences between its terminals should be minimized. The measurement system should continuously log cell current, voltage, and temperature so the three parameters can be examined over the same time interval.

Temperature-Aware Self-Discharge Measurement Solution

Perform the measurement by maintaining a stable potentiostatic voltage while continuously recording self-discharge current and cell temperature so thermal variations can be correlated with changes in the measured current. The Keysight Self-Discharge Analyzer incorporates the electrical stability and temperature measurement functions required for this battery self-discharge measurement workflow. It also incorporates a dedicated measurement port for a thermistor to measure cell temperature over time. The analyzer supports measurements on up to 32 cells simultaneously, while the Self-Discharge Analysis Software provides an interactive graphical interface for configuring measurements and analyzing logged current, voltage, and temperature. For battery temperature testing and lithium-ion cell testing, these capabilities enable engineers to examine whether variations in measured self-discharge current correspond with changes in cell temperature while maintaining the microvolt-level voltage conditions required for potentiostatic measurement.

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