battery is disconnected completely and its prompt replacement is initiated by means of a message the emergency operating capability is ensured by means of the second battery for the particularly dangerous case of a spontaneous short circuit of one of the at least two batteries the direction of current is detected which can be carried out either by means of a voltage measurement at shunts connected in series with the batteries or by means of hall probes or by measuring the contact voltage of the power switches as only the direction information is important for the actuation of the power switches by the control unit turning now to the drawings in general and in particular fig1 it shows how battery 1 can be connected directly by means of a power switch 4 or indirectly by means of a load switch 6 and the d c transformer 3 to a d c network battery 2 is connected directly by means of a power switch 5 or indirectly by means of a load switch 7 and d c transformer 3 to the d c network generation of power is preferably carried out with a generator 8 a starter 10 which is connected via a switch 11 to the d c network is provided for the operation of starting an engine further loads 9 are also connected to the system.