Battery life and performance depend strongly on temperature; thus there exists a need for thermal conditioning in plug-in vehicle applications. The effectiveness of thermal management in extending battery life depends on the design of thermal management used as well as the specific battery chemistry, cell and pack design, vehicle system characteristics, and operating conditions. We examine the case of an air cooled plug-in hybrid electric vehicle battery pack with cylindrical LiFePO4/graphite cell design and address the question: How much improvement in battery life can be obtained with passive air cooling? To answer this question, a model is constructed consisting of a thermal model that calculates temperature change in the battery and a degradation model that estimates capacity loss. A driving and storage profile is constructed and simulated in two cities - Miami and Phoenix - which have different seasonal temperatures. The results suggest that air cooling may extend battery life by 5% in Miami, characterized by higher average temperatures, and by 23% in Phoenix, characterized by higher peak temperatures. Thus, thermal management appears to have the greatest effect in regions with high peak temperatures, even if the region has lower average temperatures.