This work studied the thermal management of lithium ion batteries both numerically and experimentally. Numerically, a two dimensional CFD (computational fluid dynamics) model has been developed to perform detailed simulations of the thermal management issues within a battery pack cooled by air. Experimentally, systematic tests were performed to provide datasets to validate the CFD model. The main components in the experimental facility included a multi-cell battery pack and a wind tunnel. The wind tunnel facility generated well-controlled cooling air flow with velocity up to 30 m s−1 (∼67 miles per hour). So that the study can be performed under flow conditions directly relevant to practice. The major contributions from this combined numerical-experimental study are threefold. First, the CFD model has been shown to capture the dynamics of the cooling of battery modules consisting of multiple battery cells, including temperature non-uniformity among cells. Second, the CFD simulations have been compared directly against experimental data to quantify the accuracy and validity of the CFD models. Third, based on the validated CFD models, a reduced-order model is developed to predict the maximum cell temperature in the battery module. The accuracy and simplicity of the reduced-order model makes it promising for in situ monitoring and control purposes.