Lithium ion batteries have been extensively used in portable electronic devices and recently the focus has been shifted to more demanding applications like Hybrid and Electric vehicles. In these applications, the battery pack may consists of hundreds of individual cells, as battery sizes and usage increase, battery control and management strategies need to improve performance, safety, reliability and increase the lifecycle of the battery pack. The battery management system (BMS) needs to have an accurate online estimation of the state of charge (SOC) of the battery pack. In order to overcome this problem, this work addresses SOC estimation of Li-ion cells using both and Extended Kalman Filter and Sigma Points Kalman filter.\newline A battery equivalent circuit model has been chosen since it's a good compromise between complexity and accuracy.