its been 2 days since i haven't charged my iPhone and the usage says its only 2 hours and 40 mins and i have about 20% battery left i only made a phone call for 2 mins and dint even use the ipod or sarfai and the brightness is very low when i used the phone last time for like a couple hours i got 7 hours 30 mins on usage befor it died whats up with that ?