A vehicle may include a battery having positive and negative electrodes. The vehicle may also include a controller programmed to charge and discharge the battery according to a state of charge that is based on an effective surface lithium-ion concentration of one of the electrodes, and an effective surface lithium-ion concentration of the other of the electrodes derived from a center-to-surface lithium-ion concentration profile of the one of the electrodes. The effective surface lithium-ion concentration of the other of the electrodes may be derived via a relationship mapping the center-to-surface lithium-ion concentration profile of the one of the electrodes to the effective surface lithium-ion concentration of the other of the electrodes.