METHODOLOGICAL ASPECTS OF LITHIUM-ION NMC CELLS CYCLE LIFE TESTING

The paper considers an “accelerated” approach to experimental research, which consists in obtaining one basic degradation track and subsequent extrapolation of the results to other temperature and current operating conditions of the cell. The results of long-term cycling of a lithium-ion NMC cell produced by Soundon New Energy Technology in a current mode of 0.5C at room temperature are presented. Various methods for determining internal resistance from current-voltage measurements are discussed. It is shown that the internal resistance does not demonstrate a pronounced monotonic trend of change during cycling and cannot be used as an indicator of the state of health of cells of this type. Experimental data on capacity degradation are approximated with high accuracy by a function containing linear and exponential terms. Using the manufacturer’s data on cycling at room and elevated (45 °C) temperatures, a correction factor (Arrhenius function with an activation energy of 55 kJ/mol) was obtained, which makes it possible to extrapolate the results of the experimental study to the region of higher temperatures. Based on experimental data, a model of continuous cell degradation is proposed that can take into account time-varying current conditions and variable load patterns. The corresponding model can be integrated into the battery management module to monitor the underlying trend of capacity loss.