In this type of battery, oxygen from the air reacts with a catalytic cathode, with zinc powder acting as the anode. The main application is for powering hearing aids. A hearing aid requires a battery with small dimensions, high capacity and a flat discharge curve (leveling). Zinc-air batteries are activated after removing the adhesive foil. It is essential that this type of battery be kept hermetically sealed until use. Once the foil has been removed, the cells discharge over a period of up to 500 hours. Batteries should be stored in a dry atmosphere (heated rooms). Zinc-air batteries can be stored for an almost unlimited period of time.