It is a function if the lithium battery. It helps protect from overcharging, which would lead to shorter battery life cycle. The phone will do a rapid charge to say 99%, but will do a trickle charge to 100%. The trickle charge will increase the amount of time it takes to go from 99% to 100% to avoid the amount of times the battery will discharge and charge while on the charger. Even while on the charger, the battery can experience a slight discharge. If you've ever taken the phone off the charger and it dropped from 100% to 98%, that's what is happening.