An apparatus is provided for providing a supply voltage for operating an electrical device in a vehicle having a battery for producing the supply voltage. The battery is connected to a supply-voltage terminal for providing the supply voltage. A contacting apparatus contacts a replaceable battery for providing a charging voltage for charging the battery. In dependence on a state of charge of the battery, the replaceable battery can be electrically coupled to the main battery or disconnected from the main battery by way of the coupling apparatus.