Various embodiments of the invention provide an enclosure for a metal-air battery assembly (40) for an extended wear hearing aid. The enclosure includes a diffusion control element (80) having a dimensional property configured for controlling oxygen and moisture diffusion into the metal-air battery assembly (40) to maintain a minimum battery voltage when the hearing aid is operating and worn in an ear canal (10) of a user over an extended period. In an embodiment, the enclosure (70) can comprise a shell (75) with a base end having an opening (76) therein forming a cavity (77) within the shell (75) and a base cap (78) for covering the opening of the base end. A diffusion element (80) is disposed on the base cap. In an embodiment, the diffusion element (80) comprises a laser drilled precision micro hole having an aspect iatio of least about four and a diameter in the range of about 10 to 15 microns.