An energy storage system, comprising: a battery system including a plurality of battery racks, the battery system respectively including a plurality of rack battery management systems (BMSs), the battery racks being divided into a first group comprising at least one first battery rack including a first battery and a second group comprising at least one second battery rack including a second battery; a direct current link connected to the battery system, the direct current link comprising a capacitor; and a controller adapted to determine an operation mode of the battery system from one of a charge mode or a discharge mode based on a DC link voltage of the DC link, wherein the controller is adapted to perform a control operation in the charge mode to charge the at least one first battery rack while the at least one second battery rack is discharged and is adapted to perform a control operation in the discharge mode to discharge the at least one second battery rack while the at least one first battery rack is charged wherein the first group comprises a plurality of first battery racks, the second group comprises a plurality of second battery racks, and wherein the battery system further comprises a plurality of DC-DC converters (230a1, 230a2, 230b1, 230b2), each DC-DC converter (230a1, 230a2, 230b1, 230b2) being connected between a respective one of the plurality of battery racks (210a, 210b) and the DC link wherein the rack BMSs are adapted to determine states of charge of respective ones of the battery racks wherein the controller is adapted to control the DC-DC converters (230a1, 230a2, 230b1, 230b2) to adjust the SOC of the battery racks (210a, 210b) to be outside a first SOC range wherein outside the first SOC range lifespans of the batteries decrease at a lower rate than inside the first SOC range wherein the rack BMSs are adapted to transmit the SOC of the respective ones of the battery racks to the controller wherein, when the SOC of one of the plurality of first battery racks is within the first SOC range, the controller is adapted to control charging of the first battery rack using at least one of the second battery racks as a power source until the SOC of the first battery rack is adjusted to exceed the first SOC range wherein, when the SOC of at least one of the second battery racks is within the first SOC range, the controller is adapted to discharge the second battery rack and to charge at least one of the first battery racks with the electricity discharged from the second battery rack until the SOC of the second battery rack is adjusted below the first SOC range. a battery system including a plurality of battery racks, the battery system respectively including a plurality of rack battery management systems (BMSs), the battery racks being divided into a first group comprising at least one first battery rack including a first battery and a second group comprising at least one second battery rack including a second battery; a direct current link connected to the battery system, the direct current link comprising a capacitor; and a controller adapted to determine an operation mode of the battery system from one of a charge mode or a discharge mode based on a DC link voltage of the DC link, wherein the controller is adapted to perform a control operation in the charge mode to charge the at least one first battery rack while the at least one second battery rack is discharged and is adapted to perform a control operation in the discharge mode to discharge the at least one second battery rack while the at least one first battery rack is charged wherein the first group comprises a plurality of first battery racks, the second group comprises a plurality of second battery racks, and wherein the battery system further comprises a plurality of DC-DC converters (230a1, 230a2, 230b1, 230b2), each DC-DC converter (230a1, 230a2, 230b1, 230b2) being connected between a respective one of the plurality of battery racks (210a, 210b) and the DC link wherein the rack BMSs are adapted to determine states of charge of respective ones of the battery racks wherein the controller is adapted to control the DC-DC converters (230a1, 230a2, 230b1, 230b2) to adjust the SOC of the battery racks (210a, 210b) to be outside a first SOC range wherein outside the first SOC range lifespans of the batteries decrease at a lower rate than inside the first SOC range wherein the rack BMSs are adapted to transmit the SOC of the respective ones of the battery racks to the controller wherein, when the SOC of one of the plurality of first battery racks is within the first SOC range, the controller is adapted to control charging of the first battery rack using at least one of the second battery racks as a power source until the SOC of the first battery rack is adjusted to exceed the first SOC range wherein, when the SOC of at least one of the second battery racks is within the first SOC range, the controller is adapted to discharge the second battery rack and to charge at least one of the first battery racks with the electricity discharged from the second battery rack until the SOC of the second battery rack is adjusted below the first SOC range.