Temperature is a significant factor affecting performance and safety of energy storage systems such as battery packs. How to design a reliable battery thermal management system (BTMS) is still a hot issue at present. Most of the past researches have focused on methods of reducing temperature rise. This paper mainly studies how to reduce the temperature deviation of the battery pack while ensuring heat dissipation conditions. This paper designs a mini-channel liquid cooling BTMS with a side cover to improve heat transfer capacity and thermal uniformity in battery packs. By analyzing different side cover materials, cooling water temperature, and water channel structure, the influence of different parameters on battery heat dissipation and uniformity is obtained. The main findings are: (1) the presence of the side cover can effectively reduce the maximum temperature and temperature deviation, and the material with high thermal conductivity is more likely to dissipate heat, (2) The increase of cooling water inlet temperature can improve temperature uniformity, and (3) When the cross-sectional area is fixed, as the channel depth increases, the temperature deviation gradually decreases.