In this paper, the performance of a hybrid ultracapacitor and battery power source for onboard applications is examined. First, the behavior of the system is shown by means of a nonlinear analytical model, which is solved using a double time scale procedure; then, the performances of the source is experimentally validated through an 80-V prototype. In light of the interaction between the battery and ultracapacitor during the provision of a pulsing load, issues related to power peak, internal loss, and lifecycle are discussed. The aim of this paper is to define the design criteria for battery and ultracapacitor integration in compact and light systems.