I've been trying to calibrate my battery after enabling the battery meter by editing a .plist file in a copy of my iPod's backup. I restored that backup to my iPod, and the battery meter worked just fine. The iPod is running iOS 4.2.1.
However, attempts at calibrating it do not work. It will shut off after reaching "0%", but if I hold the power button, it just boots back into iOS with ~19% remaining.
I let it drain after that, and if I hold the power button again after it shuts off, guess what? It's back at ~19%.
I repeat this one more time before it refuses to boot into iOS. Once it drains the third time, it will boot into the Apple logo, then shut off after ~15 seconds. If I continue attempts to boot, it will continue doing this.
If I plug it into a charger though, it will boot into iOS and claim that it has 3% remaining and is charging. There are absolutely no problems with it (besides the weird battery meter).
I also own an iPhone 5 (iOS 10.3.2), and when that device hits 0, it stays there. It will not boot up and just shows the "I need to be charged" indicator.
I'd like to know what's going on here, how to get the battery calibrated correctly, and how to get the battery as stable as possible (Similar to my iPhone 5).