Hybrid Electric Vehicles (HEVs) and Electric Vehicles (EVs) are strongly emerging as potential solutions to respect the more and more restrictive laws concerning CO2 emissions. In this context, the optimization of the costly Hybrid Energy Storage System (HESS) of HEVs is a key factor in reducing both CO2 emissions and costs of HEVs. In this paper, we propose an approach to optimize the configuration of an HESS architecture using evolutionary multi-objective optimization algorithms. Multiple constraints relaxation was necessary in order to match the objectives of the study. This results in a need for a systems engineering approach to define the scope and consequences of constraints relaxation.