Dry Aid Kits and Batteries
Battery manufacturers should probably be answering this, but since I have quite a bit of experience with this I'll take a stab at it. The answer has more to do with the conditions of drying than anything else. Zinc-air batteries are designed for nominal conditions, and the acceptable temperature and humidity range is usually quite large. The allowable conditions also vary by manufacturer. Extremes, such as too high a temperature, or relative humidity that is too high or too low, can disrupt electrolyte function, making batteries fail quicker. My personal rule of thumb is that if the conditions inside the drying device are not too extreme, batteries can stay with or in the hearing aid during drying. In some parts of the world (Singapore is an example that comes to mind) a drying system with moderate temperature and humidity conditions might actually prolong battery life by removing residual moisture that upsets electrolyte balance. In New Zealand, they sell a ''high-humidity'' battery, although I have no knowledge of its effectiveness. I would probably not leave a zinc-air battery in a dry-aid kit for days or weeks, but I doubt that an overnight use would be damaging. We encourage Dry & Store users to leave the battery in with the battery door open, and user surveys show that greater than 50% report longer battery life. And a 1998 independent test by a major battery manufacturer showed longer battery life in high (80%+) humidity conditions when the battery was kept in Dry & Store for its eight-hour cycle.
A final caveat: follow the manufacturer's instructions. Five out of seven dry-aids that I randomly sampled said nothing at all about battery removal. That's a big change from when our product first came out in 1997, when almost all dry aids suggested battery removal. Perhaps some battery manufacturers will want to chime in on this discussion?