Comparison of dU/dQ, Voltage Decay, and Float Currents via Temperature Ramps and Steps in Li-Ion Batteries

Abstract

In this study, the effect of temperature changes on the voltage decay and current behavior of lithium-ion cells is investigated, focusing on a comparison between open-circuit voltage (OCV) measurements and float current (Formula presented.) measurements. Using our self-developed advanced Floater system, the voltage decay rates (Formula presented.) from OCV and float current measurements for three different cell types are assessed. Temperature ramps and steps, ranging from 5 °C to 50 °C, are applied to capture the impact of entropic effects and aging mechanisms. Both methods effectively capture aging dynamics, showing strong agreement between ramp and step measurements. Deviations arise only in cases of strong entropy effects due to differences in measurement strategies. The findings confirm that float currents do not introduce additional aging beyond that captured by OCV measurements. The relationship between OCV and float current is governed by differential capacity (Formula presented.), which varies with cell voltage and temperature. Furthermore, strong deviations from classical differential voltage analysis but high agreement with local pulse measurements are observed, especially at low depths of discharge. This can be explained by the hysteresis effect of graphite. These findings highlight the benefits of high-precision float current measurements in aging studies, particularly in contrast to simpler OCV methods.