A piecewise linear, time-varying model for modeling the discharging process of Li-ion batteries is introduced in this paper. Such a model consists of a group of piecewise linear model segments, whose parameters are adapted online with time. Thus, the combined overall model is capable of modeling a nonlinear time-varying process, such as a Li-ion battery discharging process, quite well. Modeling results of both simulated test data and actual test data gathered from a high-power automotive-grade Li-ion cell are presented. The close matches between actual and model-predicted behaviors demonstrate the effectiveness of the proposed modeling approach and indicate the potential usefulness of such models for a battery management system.