Zinc-air batteries have been in commercial use since the 1930s and are known to exhibit superior energy density and runtime compared to other types of batteries, comparatively low manufacturing cost, and safe operation. However, if stored in an open-to-air condition, the life of zinc-air batteries can be greatly effected by the humidity in the air. This limitation has resulted in primary zinc-air batteries being relegated to small market segments in which the battery operates continuously once it is placed in service. Zinc-air hearing aid batteries, of which Rayovac is currently the leading producer with 60% worldwide market share, are a good example of this type of application. A cutaway of an "Air Cell" battery is illustrated here.
Miniature zinc air batteries are primarily ed to provide power to eyeglass, behind-the-ear, and in-the-ear miniature hearing aids. In most hearing aid applications, zinc air batteries can be directly substituted for silver or mercuric oxide batteries and will typically give the longest hearing aid service of any common battery system. The general characteristics are:
Highest capacity-to-volume ratio for miniature batteries
Relatively flat discharge curve
More stable voltage at high currents than mercuric oxide or silver oxide miniature batteries
Essentially constant internal resistance
Activated by removing covering (adhesive backed tab) from air access hole
Most effective in applications that consume battery capacity in a few weeks
Must have access to air (oxygen) to operate
Excellent service maintenance prior to tab removal
Available in common hearing aid battery sizes
The electrodes in "Air Cell" batteries are gelled zinc powder anodes and catalyzed carbon cathodes. A hole in the battery container allows oxygen from the air to enter the cathode and be reduced on the carbon surface. At the same time, the zinc in the anode is oxidized in the same way as in a miniature mercuric oxide or silver oxide battery.