This is in response to ?tahW to whom you replied as well as to you.
Let’s be careful as we get technical to differentiate battery and cell … a battery has more than 1 cell. Hearing aid power sources are (technically incorrectly) called batteries; they are, AFAIK exclusively, cells.
In the zinc-air chemistry cell has a maximum theoretical output voltage of 1.65V. Design criteria of products intended for hearing aids limit it to 1.4V, thought to be for compatibility with the now unavailable mercury cells of old. This is within the 2V maximum cited in the document by ?tahW
Reference the wikipedia site which I’m required to break up with fewer than 15 posts to my credit … try en.wikipedia.org then /wiki/Zinc%E2%80%93air battery if I did it correctly
As we move to rechargeable cell chemistry, there are certainly types that are not suitable from a voltage situation. Lithium primary chemistries vary significantly, but most are over 3V. The AA and AAA have particular needs for 1.5 or so, and those exist. I don’t see any rechargeable lithium chemistries below 3V, but if the need really exists, I’d expect research to come up with something. I’d come closer to expecting a circuit that would operates at the higher voltage rather than a lower voltage cell, however. AND … secondary lithium cells are not known for safety. I’m not at all sure I’d want one between my earlobe and ear, certainly not in my ear.
We seem off the OP subject now, so let’s create a battery chemistry thread if there is a desire to go deeper.
BUT … in general, there are rechargeable chemistries that fall in the right voltage range … NiMH in particular is common and well understood by the “battery” industry.