I got a 4S just last week. I noticed this morning when pulling it off the charger (overnight) that the battery was only at 93%. Does this mean I need to re-calibrate the battery level? How is that done? Do I just need to leave it on until it drains completely and then charge it to full? I assume with a level display calibration issue that only affect is that the battery bar may not be accurate, but I'm still getting full battery.