Abstract: An apparatus for providing power to a vehicle's electric power system comprises a converter (602) for converting electric input electricity having an input voltage into output electricity having an output voltage. A sense line (612) is provided for electrically connecting a control unit (611) to the vehicle's electric power system. A power line (613) is for connecting an output of the converter (602) in parallel to the vehicle's electric power system. The control unit (611) is configured to detect a running state of an alternator (201) of the vehicle based on a measurement of a signal on the sense line (612) and control the converter (602) to set the output voltage in dependence on the detected running state.