Delete comment from: Ken Shirriff's blog
My guess is the voltage sag is intentional and helps to reduce the power dissipated in the wall charger itself. There is no need to maintain exactly 5V at the charger output when the battery is at a low charger level.As along as the sag does not extend lower than 4.35V r 4.2V at full current, the the battery will charge just fine.
Oct 29, 2012, 2:26:28 PM
Posted to A dozen USB chargers in the lab: Apple is very good, but not quite the best