I have a pretty up-to-date digital hearing aid. One of the interesting things is it has a built in capability to warn you when the battery is just about to run out.
About ten or fifteen minutes before the battery is going to die you get a special tone, like a chime. This is meant to give you enough time to change the battery. It doesn't always work that way because sometimes it goes off when you are driving or otherwise occupied. You do know as sure as anything though that you are soon to be much more deficient in the hearing department.
Thinking about it though, it is quite a capability to be able to go for a week or so and then be capable of measuring the output of the battery so you know within a few minutes when it will stop being capable of operating the hearing aid. This is my idea of one of the wonders of modern technology. It really is repeatable.