On-board electrical system for a vehicle comprising - a first on-board electrical system branch, wherein the first on-board electrical system branch comprises a first energy store, - a second on-board electrical system branch, wherein the second on-board electrical system branch comprises a second energy store, - a DC/DC converter, designed for bidirectionally transferring energy between the first on-board electrical system branch and the second on-board electrical system branch, - a first switching device and a second switching device, wherein a first terminal of the second energy store is electrically couplable to a first terminal of the first energy store via a first node, a second node and a third node by means of the first switching device and the second switching device, - a first current determining unit designed for determining a first current value of a first current flowing between the first terminal of the first energy store and the first node, - a second current determining unit designed for determining a second current value of a second current flowing via the first switching device and the second switching device, - a third current determining unit designed for determining a third current value of a third current flowing between the first terminal of the second energy store and the second node, - a fourth current determining unit designed for determining a fourth current value of a fourth current flowing between the third node and the DC/DC converter, - an electrical resistor, wherein the electrical resistor is electrically couplable to the first terminal of the second energy store by means of a third switching device, - a voltage determining unit, wherein the voltage determining unit is designed for determining a first voltage value of a voltage of the second energy store if the electrical resistor is electrically isolated from the first terminal of the second energy store and for determining a second voltage value of the voltage of the second energy store if the electrical resistor is electrically coupled to the first terminal of the second energy store, - an internal resistance determining unit, wherein the internal resistance determining unit is designed for determining a resistance value characterizing an internal resistance of the second energy store on the basis of the determined first voltage value and the determined second voltage value and for classifying the determined resistance value on the basis of the determined first, second, third and fourth current values. - a first on-board electrical system branch, wherein the first on-board electrical system branch comprises a first energy store, - a second on-board electrical system branch, wherein the second on-board electrical system branch comprises a second energy store, - a DC/DC converter, designed for bidirectionally transferring energy between the first on-board electrical system branch and the second on-board electrical system branch, - a first switching device and a second switching device, wherein a first terminal of the second energy store is electrically couplable to a first terminal of the first energy store via a first node, a second node and a third node by means of the first switching device and the second switching device, - a first current determining unit designed for determining a first current value of a first current flowing between the first terminal of the first energy store and the first node, - a second current determining unit designed for determining a second current value of a second current flowing via the first switching device and the second switching device, - a third current determining unit designed for determining a third current value of a third current flowing between the first terminal of the second energy store and the second node, - a fourth current determining unit designed for determining a fourth current value of a fourth current flowing between the third node and the DC/DC converter, - an electrical resistor, wherein the electrical resistor is electrically couplable to the first terminal of the second energy store by means of a third switching device, - a voltage determining unit, wherein the voltage determining unit is designed for determining a first voltage value of a voltage of the second energy store if the electrical resistor is electrically isolated from the first terminal of the second energy store and for determining a second voltage value of the voltage of the second energy store if the electrical resistor is electrically coupled to the first terminal of the second energy store, - an internal resistance determining unit, wherein the internal resistance determining unit is designed for determining a resistance value characterizing an internal resistance of the second energy store on the basis of the determined first voltage value and the determined second voltage value and for classifying the determined resistance value on the basis of the determined first, second, third and fourth current values.