How to make Battery Bar?

Go here http://unity3d.com/support/documentation/Components/gui-Layout.html and scroll down to the example labeled "/ Using multiple Groups to clip the displayed Contents /"

 var bgImage : Texture2D; // background image that is 256 x 32 (coud be a different size, but has to match the sizes below (GUI.BeginGroup (Rect (0,0,256,32));)
 var fgImage : Texture2D; // foreground image that is 256 x 32
 var duration = 1.0; //how long the healthbar will last
 private var time : float;
 
 function OnGUI () {
     //increase the amount of time each frame, clamp it so it doesn't go higher than duration
     time = Mathf.Clamp(time + Time.deltaTime, 0, duration)
 
     // Create one Group to contain both images
     // Adjust the first 2 coordinates to place it somewhere else on-screen
     //adjust the last 2 coordinates to change the size
     GUI.BeginGroup (Rect (0,0,256,32));
 
     // Draw the background image
     GUI.Box (Rect (0,0,256,32), bgImage);
 
     // Create a second Group which will be clipped
     // We want to clip the image and not scale it, which is why we need the second Group
     GUI.BeginGroup (Rect (0,0,(time-duration)/duration * 256, 32));
 
     // Draw the foreground image
     GUI.Box (Rect (0,0,256,32), fgImage);
 
     // End both Groups
     GUI.EndGroup ();
     GUI.EndGroup ();
 }

Give that a try and tell me if you need more help.

minor change:

GUI.BeginGroup (Rect (0,0,(time-duration)/duration * 256, 32));