The invention relates to a multi-voltage on-board power supply system for a motor vehicle having a hybrid or electric drive, comprising a dc-dc converter (DCDC), by means of which a high voltage level having a higher direct current different from earth and at least one first energy store (ES1) is connected to a low voltage level having a lower voltage different from earth and at least one second energy store (ES2). The multi-voltage on-board power supply system comprises a charging interface (S) 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 to the charging interface, and a third energy store (ES3) can be connected to the charging interface in order to supply the third energy store in the connected state with the lower voltage and to enable charging of the third energy store by energy flow from the first energy store to the third energy store.