Well mine went from 100 to 0 really quickly a few days (like in an ungodly time of 1-2 hours). It would always show up as that low battery power saving mode.
Mine's a 7. Couldn't say for when it's actually off. I responded mostly because I've been thinking my battery has been acting weird.
As for how they could die, my only thought is semi-conspiracy theory in that all devices require some level of "on" in order to keep going (the conspiracy part being that other people say that's why the devices don't have detachable batteries anymore, so they can listen in lol)... Maybe that is what is off in your case? Idk.
