Application of electric drive systems in road transportation are appealing to become practical because they offer the affordable technology that reduces vehicle emissions and fuel use, increases efficiency and enhances performance. The performance and life-cycle costs of electric vehicles (EV) and hybrid electric vehicles (HEV) depend inherently on batteries. Battery pack performance directly affects the all-electric (zero-emission) range, power for acceleration, fuel economy, and charge acceptance during energy recovery from regenerative braking. Although Li-Ion rechargeable cells have the highest energy density, and among the highest power densities, of any cell commercially available today, they are much more unforgiving of abuse than other chemistries of batteries. Safety and reliability are the top concerns of the users of electric and hybrid electric vehicles which are both subject to not only the battery technology but also the management system for the battery. For the former, battery electrical and thermal simulation models can greatly help evaluating its charge/discharge and thermal behavior and for the latter, the Battery Management System (BMS) and its cooperation with vehicle control unit should be investigated to ensure the batteries safe operation. This paper presents the design of a comprehensive battery pack simulator which consists of the electrical, thermal and the BMS models. The designed simulator results were shown to have a good agreement with real test results and therefore it can be used for predicting the battery pack dynamic charge/discharge and thermal behavior. The designed battery pack simulator has been used for Hardware-in-the-Loop (HiL) testing of the Hybrid Control Unit (HCU) in a hybrid electric vehicle. Using HiL simulations, the HCU algorithms concerning the energy storage system management are investigated. The BMS functionalities and its cooperation with HCU can also be evaluated using the designed test bench.