An on-board-unit for automatic identification of a passing vehicle in a road user charging system, said on-board-unit including: - main components for taking care of the functions of the on-board-unit including radio communication and information exchange via a communication link with a roadside equipment , - an antenna connected to the main components for the radio communication, - an identification module including a code carrier, whereby the identification module is arranged to provide a code taken from the code carrier to a signal generator which generates a response signal from the on-board-unit to the roadside equipment via the communication link identifying the passing vehicle, and - a power unit connected to the main components for supplying the required electric power to the main components , wherein the power units comprises an energy harvester and an energy storage unit , said energy harvester cooperating with the energy storage unit , wherein the energy harvester includes at least one tunable resonant mechanical device in a material capable of generating electrical power when subjected to mechanical influence, wherein the energy harvester is coupled with the power unit via an impedance and voltage matching module , wherein the tunable resonant mechanical device is: ∘ a conductor arranged for movement in relation to a magnetic field in response to an external mechanical influence, or ∘ a capacitance changing as a result of electrodes or a portion of a dielectric material moving in relation to other capacitance parts in response to an external mechanical influence, the on-board-unit being characterized in that the mechanically resonant device is tuned to have a resonance frequency within the range of 12 - 18 Hz. - main components for taking care of the functions of the on-board-unit including radio communication and information exchange via a communication link with a roadside equipment , - an antenna connected to the main components for the radio communication, - an identification module including a code carrier, whereby the identification module is arranged to provide a code taken from the code carrier to a signal generator which generates a response signal from the on-board-unit to the roadside equipment via the communication link identifying the passing vehicle, and - a power unit connected to the main components for supplying the required electric power to the main components , wherein the power units comprises an energy harvester and an energy storage unit , said energy harvester cooperating with the energy storage unit , wherein the energy harvester includes at least one tunable resonant mechanical device in a material capable of generating electrical power when subjected to mechanical influence, wherein the energy harvester is coupled with the power unit via an impedance and voltage matching module , wherein the tunable resonant mechanical device is: ∘ a conductor arranged for movement in relation to a magnetic field in response to an external mechanical influence, or ∘ a capacitance changing as a result of electrodes or a portion of a dielectric material moving in relation to other capacitance parts in response to an external mechanical influence, the on-board-unit being characterized in that the mechanically resonant device is tuned to have a resonance frequency within the range of 12 - 18 Hz. ∘ a conductor arranged for movement in relation to a magnetic field in response to an external mechanical influence, or ∘ a capacitance changing as a result of electrodes or a portion of a dielectric material moving in relation to other capacitance parts in response to an external mechanical influence, <EOS>