Lithium ion batteries are commonly employed in hybrid electric vehicles and achieving high energy density in the battery has been among the most critical issues in the automotive industry. Since thermal management is very important in the automotive batteries with layout limitation, a design strategy for effective cooling should be carefully opted. Particularly, a forced air cooling has been considered as a practical option in the automotive industry. In this article, a specific design of air-cooled battery system is theoretically investigated and numerically modelled to satisfy the required thermal specifications. Since a typical battery system in hybrid electric vehicles consists of the stacked multiple battery cells, cooling performance is determined mainly by the uniform distribution of air flow in the coolant passage which dissipates heat generated from the battery cells. It is demonstrated that the required cooling performance can be achieved by employing the tapered manifold and pressure relief ventilation even without changing the layout/design of the existing battery system. Furthermore, a theoretical analysis is performed as a design guideline to enhance the cooling performance.