In an air-cooled battery thermal-management system, inducing a large temperature difference in a battery pack is very easy because of the low specific heat of air. In this study, we develop a simple method of constructing a symmetrical air-cooled system with uneven cell-spacing distribution to improve the cooling performance. First, the influence of the battery cell number on the performance of an air-cooled system is numerically studied. The results show that the system with fewer battery cells can achieve a more uniform temperature distribution, and the energy consumption of each cell is lower. Then, different symmetrical systems are constructed based on asymmetrical systems with various inlet and outlet positions. The comparisons indicate that the symmetrical systems perform much better than the asymmetrical ones. Finally, uneven cell-spacing distribution is applied to the symmetrical systems, and the corresponding distribution expressions are derived. Compared with the asymmetrical system, we achieve at least 43% reduction in the maximum cell-temperature difference and at least 33% reduction in the energy consumption. Furthermore, the designed systems perform better than the optimized ones in the previous studies.