An energy storage device and methods for controlling the same are disclosed. The energy storage device can include a connector to or from which at least one battery pack is attached or detached, a power converter that converts external input power into DC power in a charge mode, and a switching unit that performs a switching operation such that the DC power converted by the power converter is charged in a first battery pack attached to the connector in the charge mode. Energy storage device can also include a controller that controls the switching unit to turn off electrical connections between the energy storage device and the first battery pack and a second battery pack for a predetermined off period when the first and second battery packs are attached to the connector. Accordingly, when a battery pack is attached or detached, a surge of inrush current may be reduced.