[X 2014 Accessories] Jaytn:
I wanna just jump in in case it might help someone.

TL;DR Try (see URL at end) it might or might not help, it did for me.

I had this problem since when I got the phone (maybe after ~1 month at last) - initially around 10%, rarely up to 20% (battery saver would often help, I'd just get annoyed if it died before it hit 15%). It was annoying but I didn't really bother, since most of the time I could just top it off before I leave the house and that'd be enough.

But recently it started getting worse and worse, rarely making it past 20% sometimes even dying at 30% or 35%. Then I had enough - I even searched for a replacement battery and a guide, but they're apparently not as available over here. But I didn't want to give up before trying everything; I love my phone, I don't wanna open it up and risk damaging it if I don't absolutely have to. So again I googled for "reset battery" and the like. I've tried these drain'n'load guides before and they didn't really work but I wanted to give it another shot. Then I found THIS (see URL at end) guide (scroll down to "Method 1") - it isn't really much different than the others (they all only slightly differ) but it's the one that was the first google result^^ So I tried... The battery showed weird stats while trying to drain it fully (as it does after shutting down at 30%...) but I just stuck to the guide.

And to much surprise, it worked! I started after it just had shut down at ~30% in the middle of my game (and I was pissed) and later the same day when I was done and I tested, I could use it right until 0%!

Now to the caveats (you know there are some): At least once during the procedure the battery got so low that when I plugged the charger in, it would repeatedly vibrate and show the "Loading" battery icon just to die a few seconds later again (and repeat) - for about 5-10 minutes. While this has happened before, I can't imagine that it isn't damaging the battery. Next, while I don't have any hard stats for this, perceptually I feel the overall battery life has decreased. Imagine if before it would last from 100%-30%, now that same amount would be stretched from 100%-0%, so effectively 1% being "worth" less. But then again I'd expect the capacity to decrease about 1/3 after 2 years of use, that sound's reasonable. Next, I don't think the battery levels are absolutely reliable now either. After being successful, I tried how far I could push it, and actually it remained at 1% for about 10 min while in a game, so take that with a grain of salt. Last, I don't know if the result will last. I've done it only about a week ago and have refrained from trying to drain it to 0% again, because I don't want to damage my battery even more; this phone is expected to last at least another year with Nougat ;) That being said, I've had it <25% multiple times and a couple of times below 15%, plus it hasn't failed to wake me up in the morning (because it died when the alarm went off at 25%) since :P

I intend to be more caring for my battery in the future, refraining from little "couple min charging bursts" and trying to only charge it when reasonably low and quickly unplug when full.

Now I am happy with my phone again :) and hope this helps someone, too (also sorry for the much post, very words)!

PS: Meh being lazy at posting got back at me, I can't post links. You'll have to stitch it together. Sorry for the inconvenience.

androidpit DOT com/how-to-calibrate-the-battery-on-your-android-device