Ultra-Capacitor based Hybrid Energy Storage and Energy Management for Mild Hybrid Vehicles

In a Mild hybrid electric vehicle, a battery serves as a continuous source of energy but is inefficient in supplying peak power demands required during torque assists for short duration. Moreover, the random charging and discharging that result due to varying drive cycle of the vehicle affects the life of the battery. In this paper, an Ultra-capacitor based hybrid energy storage system (HESS) has been developed for mild hybrid vehicle which aims at utilizing the advantages of ultracapacitors by combining them with lead-acid batteries, to improve the overall performance of the battery, and to increase their useful life. Active current-sharing is achieved by interfacing ultracapacitor to the battery through a bi-directional boost dc-dc converter. Furthermore, an energy management system (EMS) is developed that controls the power flow between the two sources and the load and determines the amount of charging of ultracapacitor either from the battery or during regeneration depending on the predictions based on the drive-cycle and the remaining energy of the ultracapacitor. The system is developed and implemented on a test-bench in combination with Lead-acid batteries and the performance is evaluated for step change in load. It is observed that the system has quick dynamic response. The DC-DC converter is realized using a MOSFET based converter and the control strategy is implemented using TMS320F28335 DSP.