An UPS device for electric power distribution installations, said UPS device comprising: - first electric terminals for electrical connection with an electric load to be fed by said UPS device; - second electric terminals for electrical connection with a power supply source adapted to feed said UPS device; - third electric terminals for electrical connection with a back-up unit adapted to feed said UPS device, when said power supply source is not available, said back-up unit having one or more replaceable batteries; - a power conversion stage electrically connectable with said first, second and third electric terminals, said power conversion stage being adapted to receive a power supply voltage (V A) from said power supply source or a back-up voltage (V B) from said back-up unit, when said power supply source is not available, and being adapted to provide an adjustable load voltage (V L) to said electric load; - an interface stage to electrically couple or decouple said back-up unit with or from said power conversion stage, said interface stage comprising: - a switching circuit electrically connected with said third electric terminals and said power conversion stage, said switching circuit being adapted to reversibly switch in a conduction condition, at which said switching circuit electrically connects said power conversion stage with third electric terminals, or in an interdiction condition, at which said switching circuit electrically disconnects said power conversion stage from said third electric terminals; - a controller operatively coupled with said switching circuit and adapted to command said switching circuit to switch in said conduction condition or said interdiction condition; characterised in that said interface stage comprises a feeding circuit electrically connected with said third electric terminals and said controller, wherein said feeding circuit is adapted to temporary feed said controller with a first feeding voltage (V F1) during a transient time interval following a replacement of the batteries of said back-unit with new batteries during a maintenance intervention on said back-up unit, wherein, in response to said first feeding voltage (V F1), said controller is adapted to carry out a checking procedure to check whether the new batteries of said back-up unit are sufficiently charged to feed said UPS device. - first electric terminals for electrical connection with an electric load to be fed by said UPS device; - second electric terminals for electrical connection with a power supply source adapted to feed said UPS device; - third electric terminals for electrical connection with a back-up unit adapted to feed said UPS device, when said power supply source is not available, said back-up unit having one or more replaceable batteries; - a power conversion stage electrically connectable with said first, second and third electric terminals, said power conversion stage being adapted to receive a power supply voltage (V A) from said power supply source or a back-up voltage (V B) from said back-up unit, when said power supply source is not available, and being adapted to provide an adjustable load voltage (V L) to said electric load; - an interface stage to electrically couple or decouple said back-up unit with or from said power conversion stage, said interface stage comprising: - a switching circuit electrically connected with said third electric terminals and said power conversion stage, said switching circuit being adapted to reversibly switch in a conduction condition, at which said switching circuit electrically connects said power conversion stage with third electric terminals, or in an interdiction condition, at which said switching circuit electrically disconnects said power conversion stage from said third electric terminals; - a controller operatively coupled with said switching circuit and adapted to command said switching circuit to switch in said conduction condition or said interdiction condition; characterised in that said interface stage comprises a feeding circuit electrically connected with said third electric terminals and said controller, wherein said feeding circuit is adapted to temporary feed said controller with a first feeding voltage (V F1) during a transient time interval following a replacement of the batteries of said back-unit with new batteries during a maintenance intervention on said back-up unit, wherein, in response to said first feeding voltage (V F1), said controller is adapted to carry out a checking procedure to check whether the new batteries of said back-up unit are sufficiently charged to feed said UPS device. - a switching circuit electrically connected with said third electric terminals and said power conversion stage, said switching circuit being adapted to reversibly switch in a conduction condition, at which said switching circuit electrically connects said power conversion stage with third electric terminals, or in an interdiction condition, at which said switching circuit electrically disconnects said power conversion stage from said third electric terminals; - a controller operatively coupled with said switching circuit and adapted to command said switching circuit to switch in said conduction condition or said interdiction condition;