Accurately estimating the state of charge of batteries is an essential part of battery management systems. Indeed, it conditions the ability to maintain the battery in a safe region with respect to ageing, and to ensure service availability when needed. This paper presents Bestimator™, CSEM’s novel estimation algorithm. This algorithm builds on a robust, interpretable electrical circuit model of the battery, and uses an extended Kalman filter to estimate the state of charge and correct parameters. Benchmarking against state-of-the-art estimators shows that Bestimator™ outperforms them by immediately reaching and maintaining over time an estimation error below 2%.