Recently, the introduction of the battery has been investigated in the electric power system. The purpose of the introduction of the battery is the system stabilization against the voltage fluctuation and the frequency fluctuation caused by the introduction of the renewable energy and the system accident. In this study, the battery is ranked as an instantaneous reserve power that instantaneously provides energy to the system in order to stabilize the system against the system disturbance. This study investigates the optimal battery assignment method against the system disturbance. Specifically, the one line accident in the electric power system is assumed as the cause for the system disturbance. Then, a new optimal battery assignment problem whose objective is the minimization of the variation of phase difference on all nodes caused by the accident at each branch is formulated. Then, an optimization method that consists of the tabu search and the differential evolution is applied to the formulated problem. The effectiveness of the proposed method is confirmed through numerical experiments of the application to the IEEJ EAST10 model.