The transition to sustainable development of society requires that mobility also moves in this direction. The critical issue in an electric car, in terms of energy consumption and performance, is the battery pack. In the automotive industry, there is a need for batteries with high energy density and large capacity that are safe at the same time. Lithium battery cells fulfill these requirements, if the temperature is controlled. The aim of this study is to simulate the battery thermal management system (BTMS) of a battery pack consisting of a significant number of cylindrical cells (between 1,000 and 10,000), starting from basic modules (bricks). The proposed solution has characteristics of modularity, quality, ease of integration, low cost, passive and active safety, local and remote diagnostic capacity, ease of recovery and reuse at the end of life. The BTMS considered in this paper is an air-cooling system where cold air is conveyed through lithium cells. The temperature distribution within the module of cylindrical Lithium cells is obtained by means of a Computational Fluid Dynamics (CFD) approach, where the conjugate heat transfer approach is adopted in order to find heat transfer convection between air and solid cells. Critical points are detected, where temperature peaks can occur in some conditions, such as during the charge of the battery under extreme external conditions. The sensitivity of various parameters, as the air-flow distribution and temperature, is found on the thermal performance for the proposed BTMS. The optimal air-flow conditions that give the best thermal performances for the brick are obtained. The results are generalised in order to give correlation between battery cells packing density, Nusselt number and air-flow Reynolds number.