5. ConclusionsIn this paper, we have analyzed the design attributes and performance of load-following fuel cell systems for hybrid vehicles with an energy storage device that is operated in a charge-sustaining mode. The analysis has led to the following major conclusions:The minimum power rating of the fuel cell system for this type of hybrid vehicle is determined by the vehicle power demand under sustained driving conditions.For the midsize family sedan analyzed in this work, the minimum rating is determined by the traction power at the top sustained speed rather than at 55 mph at 6.5% grade.• The architecture of the drive train may dictate the minimum cell voltage at which the stack can be operated. The capacity of the air management system determines the minimum cell voltage that can be reached.• A fuel cell system can generate more than its rated power.The maximum FCS power may be limited by the power electronics (i.e. the minimum voltage that a cell is allowed to operate at) or the by the capacity of the air management system.• Under cold start conditions the PEFC stack power is derated.For the polarization curves used in this study, the stack power is derated by 20% at 20 ◦C and by 35% at −20 ◦C.