How to Reduce Lithium-Ion Self-Discharge Test Time

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Accelerate Self-Discharge Current Measurements

Reducing lithium-ion self-discharge test time requires measuring the rate of charge loss directly rather than waiting for that loss to produce a sufficiently large change in cell voltage. The traditional delta open-circuit voltage (OCV) method monitors the decrease in a cell’s OCV while the cell remains disconnected from a load. Because lithium-ion cells have relatively low self-discharge and their voltage changes very little across much of the state-of-charge (SOC) range, the resulting voltage decrease is small. A potentiostatic self-discharge current measurement instead uses a low-noise, stable direct current source adjusted to match the cell’s OCV. The source is connected to the cell through a microammeter and maintains the cell at essentially constant voltage and SOC. As the cell continues to self-discharge, the external source progressively supplies the current needed to replace the internally lost charge. When the source and cell reach equilibrium, the externally supplied current corresponds directly to the self-discharge current. This potentiostatic measurement changes the test observable from a slowly accumulating voltage difference to current representing charge loss per unit time. Achieving shorter battery self-discharge testing times also requires minimizing electrical and cell-related settling mechanisms.

The applied direct current source must closely match the cell’s OCV before connection because any voltage mismatch changes the charge on the cell and increases the time required to return to equilibrium. The source must then remain stable at the microvolt level so voltage drift does not generate additional current fluctuations. Cell preparation is equally important. Charging or discharging creates a charge gradient within the cell, and subsequent charge redistribution produces current that decays while the internal charge distribution returns toward equilibrium. Thermoelectric voltages at connections between dissimilar metals can introduce additional errors if the two cell terminals experience different temperatures. State of charge should also be considered because self-discharge decreases considerably at low SOC and becomes more difficult to measure, while the temperature coefficient of voltage generally increases. A reasonably high SOC supports more practical measurement conditions. Series resistance provides another means of adjusting measurement response because it can be programmed to optimize settling time and sensitivity to external influences. Combining close OCV matching, stable potentiostatic voltage, appropriate cell rest, temperature control, suitable SOC, and monitored settling behavior enables self-discharge current measurement to reach an interpretable result without relying on the extended voltage-decay intervals associated with delta-OCV testing.

Accelerated Self-Discharge Measurement Solution

Perform the test by closely matching a stable direct current source to the cell’s open-circuit voltage and measuring the current required to maintain that voltage as the cell self-discharges. The Self-Discharge Analyzer is specifically designed to address the voltage stability, resolution, current measurement, and settling requirements associated with accelerated lithium-ion self-discharge test workflows. A thermistor measurement port enables cell temperature to be tracked throughout the measurement, and programmable series resistance provides a method for optimizing measurement settling time and sensitivity to external influences such as temperature variation. Self-Discharge Analysis Software provides a graphical interface for configuring and analyzing these measurements. These capabilities support potentiostatic measurement workflows where final equilibrium current or earlier population differentiation can be selected according to the required test objective.

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