The energy storage is the main issue for an electric bus operating in the metropolis such as Bangkok. In order to provide a service of at least 200 km per charge and provide enough energy for air condition in bad traffic conditions, the batteries must be installed as many as possible on the bus. However due to an increased awareness to cater for disabled and elderly customers, a low-floor bus concept has been introduced in Thailand. As a result, an installation space in a lower area of the chassis was replaced with passenger seats. Therefore, remaining space for battery pack installation could be inside the passenger room and on the roof. However, the passenger room space would likely be reserved for more seat capacity. An installation of the battery packs on the roof was considered in this study. Since the energy storage of choice was Lithium-ion batteries, an adequate cooling of battery pack was essential for using in tropical climate during both charge and discharge. In this study, ambient air was introduced for the cooling of battery packs on the roof because of simplicity of the required structure in term of design and after-service maintenance. The computational fluid dynamics was applied to simulate a heat transfer of the battery pack. Different boundary conditions were considered based on the nature of the bus operations. While the bus was operating i.e. batteries being discharged, the battery packs would be cooled by airflow generated by the velocity of the bus. On the other hand, the battery packs would be cooled by a free convection with buoyancy force in case of either parking at traffic lights or charging at station. Furthermore, model of heat sources included those from the battery cells themselves, charging or discharging, as well as the external factors such as the sun radiation. The latter could be significant especially in daytime during summer in Thailand. The cooling performance of battery pack was determined by the results of highest temperature of components inside the packs and temperature difference between each pack.