The following process is what I consider to be the gold standard for iPhone battery calibration. For some, it may not be possible to complete all of these steps.
If you’re a call attendant or hopelessly dependent on your iPhone, you may not be able to turn off your iPhone overnight or even for several hours.
At a minimum, you need to drain your battery until the device turns off, charge your iPhone to 100%, and reset it by holding the sleep/wake and home buttons until you see the Apple logo.
Use your iPhone until it turns off automatically. If your battery life is around 0% and you want to drain it faster, turn on your flashlight, turn your screen brightness all the way up, and play a video, preferably streamed from the internet.
Leave your iPhone overnight to drain the battery.
Plug your iPhone into a power outlet and wait for it to turn on. Be sure to use an Apple-supplied charger or a charger of the same capacity and amperage.
While holding the sleep/wake button, slide your finger across the “slide to turn off” screen.
Let your iPhone charge for at least 3 hours. Older iPhones should be charged within 5 hours. It does not display a charging progress indicator when your iPhone is turned off.
With the charging cable still on, press the sleep/wake button for about a second to turn on your iPhone.
When the iPhone boots up, hold down the sleep/wake and home buttons until you see the Apple logo.
If you have a newer iPhone without a physical home button, hold down the volume up button and the sleep/wake button.
When your iPhone is back online, unplug the charging cable.