NOVELTY - The method involves removing a discharged battery pack in an electric vehicle (EV) (S10). The removed discharged battery pack is transported to an electric charging/discharge line (S11). The discharged battery pack is charged (S12). Buffer capacity of the battery pack is checked (S13). The buffering capacity of the battery pack is classified (S14) into battery changing tolerant buffer battery pack. The battery pack is classified (S16) into an energy storage system (ESS) exclusive battery pack. A replacement tool of the discharged battery pack is dropped to initial position.    USE - Method for utilizing a battery pack as a power source of an environment-friendly vehicle. Uses include but are not limited to a hybrid electric vehicle (HEV), an EV, a fuel cell electric vehicle (FCEV) and a plug-in-hybrid electric vehicle (PHEV).    ADVANTAGE - The method enables charging multiple battery packs so as to replace the charged battery packs with the discharged battery pack in the vehicle, so that an operator converts a replacing mode, thus reducing accident generated during direct charging of the discharged battery pack and time to replace the discharged battery pack. The method enables separating the battery pack among the battery packs according to a buffer battery pack and classifies the ESS exclusive battery pack when charging capacity of the buffer battery pack is decreased so as to prevent discard performance of the buffer battery pack. The method enables avoiding generation electricity short during transmission of the electricity through a bus in an electric power boiler and easily utilizing electric power during emergency.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow chart illustrating a method for utilizing a battery pack as a power source of an environment-friendly vehicle.'(Drawing includes non-English language text)'    Step for removing a discharged battery pack in EV (S10)    Step for transporting removed discharged battery pack to an electric charging/discharge line (S11)    Step for charging discharged battery pack (S12)    Step for checking buffer capacity of a battery pack (S13)    Step for classifying buffering capacity of the battery pack into battery changing tolerant buffer battery pack (S14)    Step for classifying a battery pack into an ESS exclusive battery pack (S16)