In the case of a multi-voltage on-board power supply system for a motor vehicle having a hybrid or electric drive, comprising a dc-dc converter, by means of which a high voltage level having a higher direct-current voltage and at least a first energy storage device is connected to a low voltage level having a lower voltage and at least a second energy storage device, wherein the system comprises a charging interface for charging external devices, at least one converter phase of the dc-dc converter can be disconnected from the low voltage level and instead can be connected with the charging interface, and a third energy storage device can be connected to the charging interface in order to supply the third energy storage device with a lower voltage and permit a charging of the third energy storage device by the energy flow from the first energy storage device.