The on-board-unit has a switching device (D2) electrically coupled with a connector (A) in an input side to detect availability of an operational voltage provided by an external energy source (EXT) e.g. battery. The switching device is electrically coupled with a switch input (SWIN) of a switch (S1) in an output side to supply a switching signal (Z). The switching device produces the switching signal based on the availability of the voltage and a switch-off signal (ACCU-OFF). The switch-off signal is provided by a switch arrangement (CIRC) i.e. computing device, at a switch output (OFF-OUT).
