Master's Thesis from the year 2013 in the subject Electrotechnology, grade: 2.3, RWTH Aachen University (Institut für Stromrichtertechnik und Elektrische Antriebe (ISEA)), language: English, abstract: The accurate state of charge (SOC) is required for the battery for hybrid electric vehicles (HEV). Because HEV batteries, a widely fluctuating current profile; partial recharging phases (regenerative braking); inadequate lifetimes to date; expanding market and strong competition from other battery technologies is a major drawback in this application. Though as lead acid batteries are improved as well, they are widely used for micro hybrid vehicles, and the types of lead acid batteries, which are used in micro hybrid cars are quite similar to starter batteries. The motivation of this thesis work is to eliminate the drawback and determine the SoC of the energy storage system (lead acid battery) in the HEV. Because the conventional SoC determination method (e.g. coulomb counting, chemical method, voltage method etc) cannot measure the SoC correctly. In this thesis, I studied, analysed and compared different published research works and proposed a new approach by using electromotive force (EMF) to calculate SoC, which was successfully developed and validated.