The present invention relates to an energy storage system employing battery packs, comprising: battery packs; DC/DC power conversion units for adjusting magnitudes of charge/discharge power of the battery packs; an AC/DC power conversion unit for converting discharge power from the DC/DC power conversion units into AC power provided to a grid, or convert charge power from the grid into DC power provided to the DC/DC power conversion units; and an integrated control unit for individually controlling the DC/DC power conversion units on the basis of state information and additional information of the battery packs, and rated power of the AC/DC power conversion unit, wherein the integrated control unit individually controls the DC/DC power conversion units to minimize differences between full charge or discharge times of the battery packs.