change variable and Texture2D

hello guys i have create a nightvision script and that i want to make a gui battery(of nightvision machine) after the variable go to 80 or 60 and etc change the texture to another, how i can make that... take a look to my script.

 var texture : Texture2D;
 var nightVisionON : AudioClip;
 var nightVisionOFF : AudioClip;
 var isNightVision = false;
 var fAlpha : float = 0.5;
 var NightVisionLight : GameObject;
 var batteryTexture : Texture2D;
 var battery100 : Texture2D;
 var battery80 : Texture2D;
 var battery65 : Texture2D;
 var battery50 : Texture2D;
 var battery35 : Texture2D;
 var battery10 : Texture2D;
 var NoBattery : AudioClip;
 var timeSpeed : float = 0.5;
 var batteryPower : float = 100;
  
 function Start()
 {
         NightVisionLight.active = false;
 }
  
 function Update()
 {
         
         if(Input.GetKeyDown("g")&&(isNightVision == false))
         {
                 isNightVision = true;
                 batteryPower = 100;
                 audio.PlayOneShot(nightVisionON);
                 NightVisionLight.active = true;
         }
         else if(Input.GetKeyDown("g")&&(isNightVision == true))
         {
                 isNightVision = false;
                 batteryPower = 0;
                 audio.PlayOneShot(nightVisionOFF);
                 NightVisionLight.active = false;
 }
 if(batteryPower > 0){
 batteryPower -= Time.deltaTime;
 }
 if(batteryPower <= 1){ 
 (isNightVision) = false;
 NightVisionLight.active = false;
 }
 if(batteryPower <= 0.5){ 
 (batteryPower) = 99;
 }
 }
 function OnGUI()
 {
         if(isNightVision == true)
         {
                 colPreviousGUIColor = GUI.color;
                 GUI.color = new Color(colPreviousGUIColor.r, colPreviousGUIColor.g, colPreviousGUIColor.b, fAlpha);
                 GUI.DrawTexture(Rect(0.0f, 0.0f, Screen.width, Screen.height), texture);
 
 GUI.DrawTexture(Rect(43,Screen.height - 65,314,36), batteryTexture);
 if(batteryPower == 80);
 renderer.material.mainTexture = battery80;
 GUI.EndGroup();
 }
 }