The present invention relates to a battery cell which is housed in a variable state cell case in which a pole assembly having an anode/separating film/cathode laminate configuration is impregnated with an electrolytic solution. The pole assembly and the cell case are curved together in the same direction at both their ends relative to their central parts in vertical cross section in the axial direction, and they provide a battery cell having a configuration whereby they are curved in a single plane. When such a battery cell is installed in a device having a curved appearance in a single plane or is installed in a location which is curved in a single plane on the inside of a device, there are advantages in that it is efficient, can minimise waste in the space required because of the intimately fined configuration, and it allows the development of devices which have an aesthetically pleasing appearance and which have a diversity of design tailored to consumer preferences.