A hybrid control strategy is an algorithm that determines when and at what power level to run a hybrid electric vehicle's auxiliary power unit (APU) as a function of the power demand at the wheels, the state of charge of the battery, and the current power level of the APU. The design of this strategy influences the efficiency of the overall system. The strategy must balance the flow of power between the APU, the battery, and the motor, with the intent of maximizing the average fuel economy without overstressing the battery and curtailing its life.The development of a system's powertrain components and the design of an optimum control strategy for that system should be concurrent to allow tradeoffs to be made while the designs are still fluid. An efficient optimization process must involve all aspects of the system, including costs, from the beginning.In this paper, we explore the methodology behind the design of a hybrid control strategy. We also discuss the APU and battery design characteristics that are crucial to the strategy design, focusing on the interdependence of these design characteristics within the entire system. Finally, we propose a process for the development of an optimized hybrid powertrain and the corresponding control algorithm.