Accelerating float current measurement with temperature ramps revealing entropy insights

This research examines the behavior of three types of lithium-ion cells during a voltage hold, precisely measuring the necessary float current $I_{\text{Float}}$ during stepwise changes and temperature ramps from 5 to 50 °C to increase data acquisition rate and speed compared to stepwise approaches. The float current is thereby associated with calendar aging and superimposed by entropy effects in case of temperature ramps. By combining upward and downward ramps and different ramp speeds, the shares of entropy and aging can be separated. Thereby, the entropy-induced current $I_E$ across all tested cells is not constant but increases linearly with temperature. This linearity, coupled with direct proportionality between $I_E$ and the temperature ramp speed, underscores the predictability of the entropy effect in these cells under varying temperature conditions. With the knowledge of the entropy current, the aging current $I_{\text{Aging}}$ can be derived from temperature ramps and is compared with the state-of-the-art measurements during temperature steps, employing two evaluation methods. While 9 out of 12 cells show high agreement. Significant discrepancies are observed at lower voltages, particularly for high capacity NMC/NCA-cells. Kinetic effects and measurement limitations at low aging currents are expected to be the root cause, especially for upward ramps starting from cold temperatures.