A charging device is provided for an external alternating current voltage source and a motor vehicle equipped with a high-voltage battery, wherein the motor vehicle has a low-voltage battery, a link circuit capacitor and a charging electronics unit with a charging voltage input and a supply voltage input. The charging device is designed such that the vehicle has a bidirectional auxiliary power supply with two interfaces. The first interface of the auxiliary power supply is connected in parallel with the link circuit capacitor and in parallel with the charging voltage input of the charging electronics unit, and the second interface of the auxiliary power supply and the low-voltage battery are connected in parallel with the supply voltage input of the charging electronics unit.