Abstract Battery cell life depends critically on how the cell is used. Therefore, battery chargers and battery management systems must be designed to control cell usage carefully. In order to design optimal battery controls that effect a tradeoff between cell performance (in some sense) and cell life, a model of cell degradation is necessary. This model must be simple and incremental in order to be implemented by an inexpensive microcontroller. This paper takes a first step toward developing such a controls-oriented comprehensive cell degradation model by deriving a reduced-order model of a single mechanism: the growth process of the solid-electrolyte interphase (SEI) layer, along with the resulting resistance rise and capacity loss. This reduced-order model approximates a physics-based PDE model from the literature, is simple and accurate, and may be used in optimal strategies for controlling lithium-ion batteries.