A Study on Lead Acid Battery and Ultra-capacitor Hybrid Energy Storage System for Hybrid City Bus

Electrochemical energy storage, especially lead acid system alone seems not to be able to meet the demand of the hybrid vehicle propulsion system due to the power density and cycle life. Ultra-capacitor pack has been introduced in the design of energy storage system to the HEV power-train, which is often combined with battery packs to form a hybrid energy system. In these kind of applications, ultra-capacitor could either be paralleled directly with battery pack or connected with battery pack through a active energy converter like DC/DC. This paper describes two kinds of hybridization of the energy storage system with ultra-capacitor and lead acid battery. By founding the model of each component in the hybrid system and with the simulation, the power enhancement and life extension of the hybrid system compared to the pure lead acid system is analyzed. Testing results show that no matter what kind of hybridization the system is, power is enhanced with a factor β, and due to the power enhancement, current demand to the battery system is lowered, which extend the batteries' cycle life.