The spreading of electrical vehicles leads to a positive impact on the mobility field from the ecological, economical and efficiency point of view. Unfortunately this diffusion is also potentially capable of negative reverberations on distribution and transmission networks, since EVs can be considered as a new load category. Rather than preventing this virtually negative situation, it is possible for the grid to take advantage of the capabilities of EVs' batteries to implement ancillary services: for this reason the proposed vehicle's battery charger is designed to operate with bidirectional power flow (implementing V2G - Vehicle to Grid- and G2V -Grid to Vehicle- modes). The target is pursued through grid manager request towards grid connected EVs. If the power flow's direction and amount is enabled by the EV controller (depending on the SOC of the battery), the control system acquires the reference signal, which is the network sinusoidal voltage wave. This synchronization between the device and the grid is followed by the battery charger's modulation of a voltage sine wave with a certain amplitude and phase displacement with respect to the reference signal, in order to reach the desired working point which provides ancillary services to the grid. This paper analyses both the control strategy and the profiles of the transient states that take place when the switch between working points is request.