The Zinc air cell you have for you rhearing aid has a plastic label
on on side to prevent air (active element oxygen) entering the chamber
within which the zinc is held. If you look carefully under the label
you will notice a tiny hole.
As zinc is a very reactive substance in an environment it begins to
decompose releasing electricity that your hearing aid or any other
appliance can use.
So don't take your labels off before you use them!
Interestingly enough though ...
I have tested hundreds of these batteries and have noted that they still
produce current when "not activated".
T R U S T N O O N E