A state of charge of a battery is estimated by a battery model specific to the battery. The battery model provides a section-wise defined correlation of terminal voltage values depending on state of charge values. Each of sections of the battery model delimits a monotonic dependence of the correlation from others of the sections. By segmenting the correlation of terminal voltage values depending on a state of charge value into sections, each segment within such the section-wise defined correlation of terminal voltage values depending on state of charge values is mathematically spoken a bi-unique function suitable for transformation into an inverse function defined within the section.
The estimated state of charge may be continuously refined by an iterative feedback loop including coulomb counting for estimating a battery charge value, where refined estimated battery charge values state of charge values at a previous cycle are projected forward to the current cycle.