The expected rollout of plug-in Electric Vehicles (EV) in the next few years will generate additional constraints to the electrical power systems if badly managed: demand increase and risk of peak loads. Nevertheless, these EV could also be an opportunity thanks to their storage batteries. Vehicles could then be harnessed to provide services such as peak reduction or frequency regulation. Previous studies have already identified profit opportunities mainly through frequency regulation. The economic gain will be only possible, however, if the technical response of EV fulfill the technical requirements for frequency control participation. This paper evaluates different scenarios to analyze how an EV fleet could contribute to primary control, how it could respond to frequency variation and, finally, how to set up an optimal frequency control that respects the battery lifespan. As a first step towards consideration of a fleet of vehicles, the results presented in this paper will focus on a single vehicle.