A battery energy storage system comprising: a plurality of battery boxes provided with a plurality of battery packs 111a); a plurality of power converter systems respectively connected to the plurality of battery boxes to selectively charge or discharge the plurality of battery boxes through alternating current supplied from an external power source device; a transformer disposed between the external power source device and the plurality of power converter systems; and a controller configured to control the plurality of power converter systems to selectively charge or discharge the plurality of battery boxes, wherein the plurality of power converter systems is connected to the plurality of battery boxes in parallel, characterized in that the controller is configured to check a normal or abnormal state in real time through a detecting sensor mounted on the power converter systems or the battery packs when charging or discharging the battery packs, wherein the detecting sensor is a temperature detecting sensor or a smoke detecting sensor, wherein the controller is configured to control a monitor device to display a connection state between the plurality of battery boxes and the plurality of power converter systems through the monitor device, when the plurality of battery packs are connected to each other in parallel, the battery box is configured to increase in capacity, and when the plurality of battery packs are connected to each other in series, the battery box is configured to be able to increase in voltage, wherein the controller is configured to control the plurality of power converter systems to charge or discharge only the battery box having the high capacity and also charge or discharge only the battery box having the high voltage. a plurality of battery boxes provided with a plurality of battery packs 111a); a plurality of power converter systems respectively connected to the plurality of battery boxes to selectively charge or discharge the plurality of battery boxes through alternating current supplied from an external power source device; a transformer disposed between the external power source device and the plurality of power converter systems; and a controller configured to control the plurality of power converter systems to selectively charge or discharge the plurality of battery boxes, wherein the plurality of power converter systems is connected to the plurality of battery boxes in parallel, characterized in that the controller is configured to check a normal or abnormal state in real time through a detecting sensor mounted on the power converter systems or the battery packs when charging or discharging the battery packs, wherein the detecting sensor is a temperature detecting sensor or a smoke detecting sensor, wherein the controller is configured to control a monitor device to display a connection state between the plurality of battery boxes and the plurality of power converter systems through the monitor device, when the plurality of battery packs are connected to each other in parallel, the battery box is configured to increase in capacity, and when the plurality of battery packs are connected to each other in series, the battery box is configured to be able to increase in voltage, wherein the controller is configured to control the plurality of power converter systems to charge or discharge only the battery box having the high capacity and also charge or discharge only the battery box having the high voltage.