Abstract: A back-up device includes at least one back-up battery, in particular a rechargeable battery which can be charged via the power supply, and a control unit. The control unit connects the back-up battery when the actual value of the DC voltage is less than a predefined minimum value, generates a predefined back-up time period and disconnects the back-up battery again, cyclically, after this time period has elapsed. The device has the advantage that the return of the output DC voltage of the power supply can be identified effectively without major complexity. The loads continue to be supplied both in the event of a failure of the supplying input voltage and in the event of a failure of the power supply, since the output DC voltage, which is essential for the loads, of the power supply is monitored. The device can be connected additively downstream of a power supply, without any intervention in its design being necessary.