The other day, I decided to plug in my old Thor model HDX7, since I couldn't remember the last time I had bothered to charge it. Was surprised to find it with 6% remaining. But, my relationship with it regarding battery life has been kind of like the Energizer Bunny commercials with "Going, and going, and going"; or whatever it is they use as a slogan. I don't think I ever managed to really ding its battery, aside from leaving it idle for a couple weeks.
Extra surprising: the last update it got for Fire OS was 18.104.22.168 installed February, 2019.
That's really not bad for a device I got free from a Dolby developer raffle. Still running a form of Android 4.4/K, but considering the device came out around 2013, and had internals on par with the Nexus of the day—I'd say the little thang has faired pretty well.
I suppose one of these days, I should probably finish turning it into a digital picture frame, or something.