Growing demand for mobile energy storage systems makes it essential to ensure the safe operation and the elongation of the service life of such storage systems. Today’s most common mobile energy storage system is an electrochemical cell based on lithium. The so called lithium ion battery requires a strict operation window in terms of terminal voltage, load current and surface temperature. Battery management systems (BMS) deal with solutions to ensure the safe operation of lithium ion batteries. The functionality of such a BMS includes also to estimate the state of the cell and to provide some information (mostly the state of charge) to the user. The state indication is of high importance due to the fact that the knowledge about the health of the battery enables the BMS to act if the battery health gets worse. This book contains the description of measurement strategies to identify the state of charge (SOC) and the state of health (SOH) of a lithium ion battery. The parameters figured out will enable a suitable battery model (i.e. electrical equivalent circuit) to calculate those state estimation factors.