A hearing aid device (100, 100a-b) comprises a shell portion (104) and a battery door module (200, 200a-b) attached to the shell portion. The battery door module comprises a battery casing (221, 221a-b) and an electronics casing (208, 208a-b). The battery casing receives a battery (202) defining a battery axis (LBAT) that extends substantially perpendicular to a longitudinal axis (L) of the battery door module. The electronics casing encloses a voltage regulator (216), a printed circuit board assembly (214), and at least a portion of one or more charging elements (220, 220a). The battery door module also comprises at least one attachment member (204, 206, 206a-b) that allows the battery door module to attach to a shell portion of the hearing aid device. The battery door module is movable relative to the shell portion between an open position and a closed position.