Battery-management systems (BMS) seek to maximize battery-pack performance while extending life by optimizing dynamic operational power limits. To do so in a meaningful way, the BMS must have computationally simple yet accurate models of degradation to use in the optimization. As the growth of a solid-electrolyte interphase (SEI) layer on negative-electrode particles in lithium-ion cells is the dominant aging mechanism leading to capacity loss and resistance rise, we propose a reduced-order model of diffusion-or kinetics-limited SEI growth that could be used in an embedded optimization framework. This model approximates a full-order partial-differential-equation (PDE) model in the literature with less than 1 % relative error, at a speedup of more than 19 000 : 1.