Embodiments include a displacement-generating battery cell for driving a drug-delivery device. The cell includes at least one volume-changing element. The cell also includes a housing formed according to a concertina-shaped design with folds in the walls thereof and containing an internal chemical reaction system. The arrangement of the chemical reaction system is such that as the cell is discharged, the volume-changing element expands, thereby lengthening the battery and thus reducing the extent of said folds, such that the cell becomes taller to reflect the expansion of the volume-changing component.