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,