Chosen Solution
Replaced battery on iPhone 6. Battery was at 24% when finishing the repair. Used it until it went below 10% per the calibration procedure. Turned phone off and plugged in. After four hours percent charge only came up to 16%. Since then have been using Phone and it will not charge above 9% regardless of how long it’s plugged in.
@mnackman try this. I am suspecting your iPhone immediately comes back to life when you plug in the charger. If it does, unplug the charger. Use the phone until it powers off itself. Repeat plugging in the charger briefly to allow the phone to start up and then the phone naturally power up. When the phone no longer comes back immediately, then allow it to charge to full. This should fix your issue with calibration. If not, the battery likely is defective and should be exchanged or returned.
