The only time the battery is not being used at all is when it's not in the phone. (Technically it still is, since batteries discharge themselves just a bit even when not connected to anything.) You have radios (2G/3G/4G, wifi, Bluetooth) running and they consume battery. Apps running in the background consume some battery. Etc. That's why there are two battery times in a phone's specs - talk time and idle time. And the more radios you have turned on and the more apps you have running, the shorter the idle time.