Don't forget that you have first to configure the scene file for batch rendering. Namely, you have to choose the option "render to disk" and you have to define the file where the render will be saved. You also have to choose "external renderer" in the render options. Save the scene file and then you can apply it to the batch rendering engine. I normally save the file, exit Vue, open Vue again, do not load the scene and then open the batch rendering, as Crowning explained, and apply the scene. This way it uses less memory.