A supply circuit has a first and a second terminal for connecting an accumulator, a third and a fourth terminal for connecting at least one battery, and an output terminal. A voltage regulator is connected to the first terminal on the input side and to a fifth terminal on the output side. An undervoltage detection circuit is adapted to activate the voltage regulator when a voltage at the first terminal is greater than a threshold voltage. A reverse polarity protection device is coupled between the third terminal and the output terminal. A blocking diode is coupled between the fifth terminal and the output terminal.