I don’t think a battery app for replaceable batteries, would be of much use, zinc air batteries, tend to hold their voltage well, then there is a rapid decline… I have had physical battery checkers in the past, but if I am being honest, they could give me a voltage reading, but not the amount of juice still left in the cell… My Phonak Naida Lumity & Paradise aids, with 675 batteries, give me a ample warning beep, approximately 1 hour prior to them dying, I always have a spare 6 pack on me, I will sometimes just let them die, if I am working on my own or there is no one else around… I normally get 11 to 12 days runtime from the 675’s, I usually keep an eye on the batteries, normally I will mark my iPhone calendar, so I am aware when they are about to die… In the UK, most hospitals have a, disposable hearing aid battery recycling facility, where you can drop off your used batteries… Occasionally, if I have something on, I will preemptive a batteries change, especially if I am around 11 days since my last batteries where changed
Cheers Kev 