Systems and methods for accurately determining the state of health (including state of charge and relative age) of a Lithium Sulfur battery, module or cell. The invention uses an operational model of a Lithium Sulfur cell or battery to predict model parameters under a range of conditions related to state of charge and state of health. Operational models include the memory effect due to the unique chemistry of a Lithium Sulfur cell that precludes the user of other methodologies for State of health determination for Lithium Sulfur batteries. Model parameters are identified in real life applications and parameters are compared to those of the operational Lithium Sulfur model employing Kalman filtering. The output includes an estimate of state of health and other key performance indicators. Key performance indicators are compared with measured values of for example resistance to provide feedback to the estimate process in order to improve accuracy. The system can be implemented as software or firmware in an application.