In this paper, we present a completed Hybrid/Plug-in Hybrid Electric Vehicle (HEV/PHEV) model including an Electrified Auxiliary System (EAS), and an impact analysis of this auxiliary system on the vehicle energy efficiency. Our approach focus-es only on three auxiliaries (an electrified air supply system, an electrified refrigerator, and an electric hydraulic power steering) but could be applied on other devices. Numerical simulation shows that this system has a high impact on the energy efficiency of the HEV/PHEV, while its energy consumption could reach up to 18.6% of the total energy requirement for vehicle propulsion. For the purpose of improving the vehicle energy efficiency thank to the electrified auxiliary management, an optimization approach based on forward dynamic programming is developed to determine the optimal auxiliary power consumption profile. The results of this work conduct to some prominent suggestions for the design of the electrified auxiliary management.