A chargeable battery or a battery pack stores electrical energy, and
supplies the electrical energy to a portable terminal device connected to
the battery pack. A display device of the battery pack derives a value of
a current flowing through a sensor which is connected to the chargeable
battery. The display device derives the remaining amount of electrical
energy stored in the chargeable battery, by adding up the derived current
value. The display device includes a plurality of photodiodes, and shows
the remaining amount of electrical energy stored in the chargeable
battery, by lighting up corresponding number of photodiodes corresponding
to the derived remaining amount.
