High-Efficiency Adaptive-Current Charging Strategy for Electric Vehicles Considering Variation of Internal Resistance of Lithium-Ion Battery

This paper outlines a battery charging strategy to reduce charging losses in a lithium-ion battery for electric vehicles. The proposed charging strategy utilizes an adaptive current profile based on variations of the battery internal resistance as a function of the state of charge and the charge rate. To address the problem of finding the optimal current set for the proposed strategy, an evolutionary algorithm, which is a type of stochastic approach, is applied. A strategy for selecting the optimal number of charging intervals is also presented to reach a compromise between loss reduction and computational burden. Experimental results obtained using 34-Ah lithium-nickel-manganese-cobalt-oxide battery cells have proved that the proposed charging strategy decreases the charging loss of the battery cell by 40.1% compared with a conventional constant-current charging strategy. Furthermore, a 3.3-kW on-board charger prototype has been built to investigate the total loss reduction in an electric vehicle charging system that includes a 12-kWh battery pack. The proposed adaptive-current charging strategy reduces the total charging losses including both battery loss and charger loss of electric vehicles by 7.2%, 11.2%, and 21.2% in charging systems with power ratings of 3.3 kW, 6.6 kW, and 13.2 kW, respectively. These improvements can have the same effect as increasing the charger efficiency from a minimum of 0.50% to a maximum of 3.72% in our analysis range of 3.3-52.8 kW.