The goal of this paper is to investigate the application of nonlinear control technique to a multi-input multi-output (MIMO) nonlinear model of a wind energy battery-storage system using a permanent magnet synchronous generator (PMSG). The challenge is that the system should operate in both grid-connected and standalone modes while ensuring a seamless transition between the two modes and an efficient power distribution between the load, the battery and the grid. Our approach is different from the conventional methods found in literature, which use a different controller for each of the modes. Instead, in this work, a single unified nonlinear controller is proposed. The proposed control system is evaluated in simulation. The results showed that the proposed control scheme gives high dynamic responses in response to grid power outage and load variation as well as zero steady-state error.