For some additional reference I just replaced the battery in my HTC Flyer. I bought it at Best Buy in October of 2011 as they had a sale marking them down from $500 to $300. A lot of cash but I felt a strong need for a reward for hanging on at a soul sucking job long enough to qualify for a (small) pension. Best Buy probably received the unit in May, that's when they got them, it had been stashed away in a drawer when I bought it.
It's gradually been losing its ability to hold a charge for the last two (or more) years, but I could still get a day (6-8 hours of steady use) out of it. It's just that even when shut off it would drain a lot, or when used sparingly it would drain faster than it used to. Battery is supposedly not replaceable, but with a T5 Torx screwdriver, and patience, I was able to take it apart and then deal with the delicate work of unplugging/re-plugging three delicate connectors to the battery. Pulling the case apart was less of a hassle. Lol, the how to video on this was quite good, but only when I got down to it did I notice that they used a lot of zoom, and the person doing it did not have large hands.
It was an OEM HTC battery (4000 mAh) I got from Amazon for around 20 bucks, I needed the screwdriver but I got a sweet deal on a nice set for eleven bucks.
So now I'm back to playing with my revitalized Flyer. I celebrated my success by finally buying a N-Trig pen for it. After a lot of detective work I tracked down a near exact replacement for the HTC one for it for twenty bucks. And I scored a new HTC OEM cradle for it for s$6.48, tax included. I've got an new Amazon Fire HD 8 which is more powerful, and I even put the Play Store side loader on it, but there's something neat about the Flyer. It has excellent audio, a nice balance, and HTC Sense (though it's stuck at Honeycomb being the last supported release.)
Anyway, the battery is working nice!
https://www.amazon.com/gp/product/B074J7TFQC
Technically I got over six years out of the last one, though it's seen scarce use over the last couple of years. I think the battery was installed in the Flyer before May of 2011. Now I have a Moto G (original edition) and a Moto G5+ in addition to the Flyer. The Moto G and the HTC are both WiFi only. Though I bought Groove IP Pro so I have a phone number for WiFi phone calls as a backup for my cell phone. I had an Amazon HDX but the screen cracked.
Btw, I highly recommend Binaural Beats at the Play Store. I keep my Flyer as the player of that. It has excellent stereo speakers for a tablet, and excellent output to my headphones. Amazingly effective.
https://play.google.com/store/apps/details?id=com.github.axet.binauralbeats
P.S. I almost always used a regular charger on the Flyer, and not the fast charger it came with. I'm carrying that over to my Moto G5 even though it came with a sweet quick charger, and it's set up to charge up very quickly when using that. I think that might help longevity, keeping the battery from getting hot. I could go on and on about my revitalized Android pal ...