The present teachings are directed toward machine implemented method for estimating the ion density of the surface of either positive or negative electrode of a battery. The machine-implemented method includes dividing each electrode into N layers of active electrode material, determining the ion density variable for each one of the N layers of the active electrode, and determining the ion density of the electrode surface. In the presently disclosed method, the ion density variable of each of the N layers of the active electrode changes as a function of the difference between the respective ion density variables of adjacent N layers, and the ion density of the electrode surface changes as a function of the battery current and the difference between the respective ion density variables of adjacent N layers. The present method is particularly applicable to Li-ion batteries.