Soooo.... I installed fresh batteries in my aids this morning. If the batteries are going to last, on average, 7 days, then I should be down by a bit more than 14% tonight - in other words, my Oticon apps should indicate 86% charge remaining. Both hearing devices are indicating 100% power remaining. Both of them show 100%.
Wassup wit datt? (This is just the second set of batteries into the new devices. Do the aids have to go through several 100%->0% cycles in order to calibrate themselves?)
I think this goes back to what
@e1405 was saying about the inability of the battery level estimator on the hearing aid to make it accurate enough, especially in the first few days of usage. It simply measures the voltage level of the battery and tried to convert that into remaining charge estimate, but as you see below, the voltage curve for the 312 Zinc-Air battery over time at a 1 mA discharge rate is fairly flat until it almost precipitously falls off the edge of the clip (except for a few on display here). While there's a very slight slope for most of them that should help with the estimate, the very slight slope doesn't lend to help give it enough accuracy.
As for Lithium-ion batteries, below is an example voltage curve over time for it at a constant discharge load. Note however that this is not for a typical Lithium-ion HEARING AID battery because it cannot supply a 2000 mA load like in this example (which is 2000x compared to the discharge load of 1 mA in the chart above for the 312 size Zinc-Air). But the characteristic of the Lithium-ion battery scaled to the hearing aid size should not be that much different anyway. You can see that with the Zinc-Air battery above, the slope is about maybe around 0.2 volt drop only over its life cycle. Compared that to the Lithium-ion 0.7 volt drop over its life cycle, a 3 to 4x difference -> easier to track remaining capacity with a relatively steeper slope. On top of that, the Lithium-ion hearing aid battery gets the majority of its full charge cycle used up within 1 day time, so it makes it easier to combine the voltage measurement with some guesstimate based on the to-the-minute time used during the day, for a more accurate total estimate. The Zinc-Air, on the other hand, usually lasts over the course of 4-5 days, making it much harder to add in a guesstimate to combined with the actual voltage reading for better accuracy.
The bottom line is that most people with disposable Zinc-Air batteries don't care so much on how much is left on their battery anyway because if one runs out, you just pop a new one in without worrying about being SOL because it takes time to charge the spent battery up. There's no longer a need to be obsessed over the remaining charge of your battery because premature out-of-battery anxiety doesn't exist like with Lithium-ion batteries anymore. So over time, they can kind of estimate for themselves how many days their batteries last depending on usage, and that's good enough for them. For example, if they can guess that their disposable lasts between 4.5 to 5 days based on their typical usage, for example, then they can decide to just change out to new batteries after 4 days if they don't want to be caught with a dead battery in the middle of the day. Those who want to squeeze out every mA of their disposable battery will simply always carry spares with them on the go and just pop a new one in as the old one runs out.