data which indicates remaining capacity of a battery in terms of the terminal voltage of the battery is updated by a control unit when necessary , whereby available capacity of the battery is accurately calculated regardless of the extent of deterioration of the battery . in addition to the battery &# 39 ; s calculated available capacity , a radio - wave receiving condition and a temperature detected by a temperature detection unit are also taken into consideration to accurately calculate and display an available time . moreover , the control unit performs the entire process of backing up user data into a non - volatile internal memory , which reduces power consumption required for the backup process , resulting in a longer available time .