Charging Optimization Methods for Lithium-Ion Batteries

Abstract

Traditional charging technology uses external battery parameters, e.g., terminal voltage and current, as the control target, and only controlling external parameters does not give information on internal characteristics of the battery, and thus, the effects of different charging currents and cutoff voltages on battery degradation are not clear. In this chapter, the electrochemical reaction mechanisms and external characteristics of the battery during charging process are studied, and the mechanisms of battery charging performance and characteristics of charging polarization are revealed. By researching the electrochemical reaction law and potential distribution characteristics of the battery during the charging process, a novel electric model based on the Butler–Volmer equation was employed to outline the unique phenomena induced by changing rates for high-power lithium batteries. The robustness of the developed model under varying loading conditions, including galvanostatic test and Federal Urban Dynamic Schedule (FUDS) test, is evaluated and compared against experimental data. The analysis of polarization voltage features at different charging rates indicates that polarization voltage is high on both ends of the SOC range but low in the middle SOC range, and the shape of the polarization voltage curve is like a bowl. In the middle SOC range, an approximate linear relationship exists between the steady-state polarization voltage and the charging rate. The two time constants (TCs) representing polarization voltage change are in 10- and 1000-s orders of magnitude, respectively, which corresponds to three charging reaction processes. The dynamic polarization voltage exhibits a lagged effect and an overshoot effect when the charge current is changed. Depending on the polarization voltage characteristics, setting battery polarization voltage and charging cutoff voltage as the constraint conditions, the calculation method for the maximum charge current of a Li-ion battery based on the battery polarization time constant is established, which can help engineers design a practical charging strategy. An optimal charging strategy is devised to balance charging time and temperature rise, with polarization constraints fulfilled. The charging target function is constructed by setting limits to the charging temperature rise and shortening the charging time as the optimization target. The optimal charging current curve is determined by the genetic algorithm (GA) under the constraint of the maximum charge current and limited by polarization voltage. The experimental results indicate that the developed charging protocol can reduce charging time remarkably with reasonable temperature rise, highlighting its advantages over conventional CC–CV charging methods. Aging experiments further verify that the developed charging protocol has a similar capacity retention ratio, compared to that of 0.5C CC–CV charging after 700 cycles. By effectively combining the external characteristics and the internal electrochemical reaction during the charging process, the optimized charging strategy with polarization voltage as the control target results in a fast charging process without damage to the battery life.