A power supply system (2) for vehicles, comprising an electric machine (6), a DC-AC converter (8) having a DC connection and an AC connection, and an energy storage element (4) wherein the energy storage element (4) can be connected to the DC connection and the electric machine (6) can be connected to the AC connection, wherein the electric machine (6) can be operated using the energy stored in the energy storage element (4), characterised in that a DC-DC converter (14) is arranged between the energy storage element (4) and the converter (8) and an external power supply (12) can be coupled into the DC-DC converter (14) for charging the energy storage element (4) wherein the DC-DC converter (14) can be uncoupled from the converter (8) in the event of power supply.