Download 1. Introduction Throughout the years hybrid vehicles have proofed themselves the shorter path to efficient, non-contaminating transportation. While new and conventional technology batteries are still on their way to achieve a reasonable energy density, which could make pure electric transportation a competitive option, the combination of high-energy-density primary sources and high-power-density auxiliary sources have already reached the automotive market in the form of attractive, low-operational-cost hybrid models. This trend has encouraged technology developers and researchers within the electric traction field to explore new auxiliary energy systems, which combine optimal energy management and high-power-density storage devices (Flywheels, Ultracapacitors, high-power batteries, etc.). In the context of developing a series topology hybrid vehicle, an Ultracapacitor-based Auxiliary Energy System (AES) has been implemented and tested. This paper presents the results of experimental drive tests using the AES in combination with lead-acid batteries as a primary source. The purpose of these tests is to asses the increase of the overall efficiency in the use of energy, using a primary source that is clearly inefficient at high power regimes. The results will lead to conclusions about the adequacy of this particular AES (Ultracapacitors and a Buck-Boost converter) for …..