gas at a fairly controllable rate, proportional to the load. This can build up pressure, which is used in certain applications to expel another liquid over a longer period, like automatic lubricators or air fresheners. Alternative configurations Attempts to address zinc–air's limitations include: Pumping zinc slurry through the battery in one direction for charging and reversing for discharge. Capacity is limited only by the slurry reservoir size. Alternate electrode shapes (via gelling and binding agents) Humidity management Careful catalyst dispersal to improve oxygen reduction and production Modularizing components for repair without complete replacement Safety and environment Zinc corrosion can produce potentially explosive hydrogen. Vent holes prevent pressure build-up within the cell. Manufacturers caution against hydrogen build-up in enclosed areas. A short-circuited cell gives relatively low current. Deep discharge below 0.5 V/cell may result in electrolyte leakage; little useful capacity exists below 0.9 V/cell. Older designs used mercury amalgam amounting to about 1% of the weight of a button cell, to prevent zinc corrosion. Newer types have no added mercury. Zinc itself is relatively low in toxicity. Mercury-free designs require no special handling when discarded or recycled. In United States waters, environmental regulations now require proper disposal of primary batteries removed from navigation aids. Formerly, discarded zinc–air primary batteries were dropped into the water around buoys, which allowed mercury to escape into the environment. See also List of battery types List of battery sizes Comparison of battery types Aluminium–air battery Fluidic Energy Fuel cell Gas diffusion electrode Hydrogen technologies Metal–air electrochemical cell Zinc-bromide battery References External links Further reading Heise, G. W. and Schumacher, E. A., An Air-Depolarized Primary Cell with Caustic Alkali Electrolyte, Transactions of the Electrochemical Society, Vol. 62, Page 363, 1932. Fuel cells Metal–air batteries Battery types Zinc