The overall efficiency of battery energy storage systems (BESSs) strongly depends on the temperature uniformity of the batteries, usually disregarded in studies of the integrated performance of BESSs. This paper presents a new battery thermal management system (BTMS) using a personalized air supply instead of a central air supply. Thermal models are established to predict the thermal behavior of BESSs with 400 battery packs. Moreover, several optimizations comprising the effect of the position and number of air inlets, the number, and angle of the baffles on the air distribution in the ducts are proposed. The results show that the distributed air supply from the main air inlet makes the air velocity in the main air ducts more uniform. It is demonstrated that air deflection is the main source of airflow inhomogeneity at the air outlets. The airflow uniformity is better when the baffles are added at the entrance and the bottom of each riser duct than at other locations. The improved air supply scheme makes the nonuniformity coefficient of air velocity reduced from 1.358 to 0.257. The findings can guide the selection of a cooling form to enhance the safety of BESSs.