In this paper we develop a nonlinear coordinated controller for a battery storage and a generator excitation that guarantees terminal voltage regulation and transient stabilization of the generator. The proposed controller is based on a new state transformation that enables the derivation of a Multi-Input Multi-Output (MIMO) feedback-linearizing controller. First, we define the state-space transformation that transforms the original state-space into a new state-space that contains all the relevant state-variables for achieving transient stability and voltage regulation. Then, we design the coordinated controller for the energy storage and the generator excitation based on this tranformation and MIMO feedback linearization theory. We illustrate the achievement of the desired objectives by the controller through simulations on a 3-bus power system.