Hybrid energy storage system and energy distribution strategy for four-wheel independent-drive electric vehicles

Abstract

This paper presents a novel topology of a hybrid energy storage system (HESS) and an improved energy distribution control strategy for four-wheel independent-drive electric vehicles (4WIDEVs) to improve their energy efficiency and dynamic performance under urban driving conditions. The small 4WIDEV was developed for only urban driving conditions and was limited to a maximum speed of 60 km/h. The HESS consists of a battery, four ultracapacitor (UC) groups and five direct current/direct current (DC/DC) converters. The 4 UC groups of the proposed HESS have two connection modes under braking conditions, namely a series-connected recovery structure (SCRS) and a parallel-connected recovery structure (PCRS). By changing the voltage ratio between the low-voltage and high-voltage sides of the DC/DC converters, the conversion efficiency can be improved, and thus, the braking energy can be effectively recovered. The upper and lower limits of the state of charge (SOC) of the UC are estimated according to the vehicle speed, and associated energy distribution rules are established to achieve a reasonable energy distribution between the battery and the UC. The proposed composite power system and energy distribution strategy were simulated, and a hardware-in-the-loop experiment was carried out on the experimental vehicle. The simulation and experimental results show that under urban driving conditions, the HESS can fully utilize the characteristics of UC, charging and discharging quickly with large current; to meet the instantaneous high power demand of electric vehicles, improve the power performance of electric vehicles, and protect the battery from the impact of large currents. The braking energy recovery efficiency of the vehicle increased by 13.53% in the simulation and by 10.26% in the experimental vehicle test.