The battery state of charge (SOC) is an important parameter of the battery capacity state. It can only be indirectly estimated through measurable variables such as voltage, current and temperature. Accurate estimation of SOC is one of the key problems in a battery management system. A battery model based on equivalent electrical circuits has been used to describe the battery dynamics. The model has been experimentally validated using a laboratory test. The battery SOC has been estimated in real time by means of two methods: Luenberger observer and Kalman Filter. This paper presents a comparison between the two model based SOC estimation algorithms.