Optimal thermal management of Li-ion batteries is one of the key elements to accelerate the further development of Electric Vehicles (EVs) and Hybrid Electric Vehicles (HEVs). The growing demand and development of EVs and HEVs requires long battery operation time and high power to satisfy the customer requirements. A desired battery thermal management system must provide an optimal temperature range to guaranty its safe operation and service life. Battery performance such as power and energy capability, life cycle and charge acceptance can be controlled by the operating temperature. Accordingly, a new Li-ion battery cooling architecture has been developed between the University of Luxembourg and Mahle, Germany. In the proposed concept, the battery cells are in direct contact with a two-phase dielectric fluid. The cooling system uses the heat of vaporization of a dielectric fluid to control the temperature of the cells. The present project aims to experimentally and numerically investigate the flow boiling mechanism governing the heat transfer process between the working fluid and the battery cells as well as the heat discharge to the ultimate heat sink – ambient air. Furthermore, the obtained results will demonstrate the improvement of heat transfer and temperature distribution within the battery pack of the proposed cooling architecture.