My battery is not getting charged above 80%
I bought this phone a month ago and I am facing this issue from last 2 days
As far as I know, the battery calibration thing is a myth especially with the batteries currently made today. If I'm wrong I would like to see some articles/ tests that prove otherwise.
As for the battery, I would recommend going to Apple and get the unit exchanged. Not to jinx you or anything, but make sure you keep that phone super secured. Apple might fight back if there are any dents/cosmetic damages on the unit.
If it's under warranty the best thing to do is to take it back for a new one.
But you can try running the phone down to zero (or as close as you can get) and try a full charge. Sometimes the battery monitoring chip needs to go all the way down so it can calibrate.
This is usually caused by the battery getting too warm. The official answer from Apple is "Your iPhone might get slightly warmer while it charges. To extend the lifespan of your battery, if the battery gets too warm, software might limit charging above 80 percent. Your iPhone will charge again when the temperature drops. Try moving your iPhone and charger to a cooler location."
Hope this helps!