This is a question many ask and popular opinion would answer in the affirmative. The truth, however, is charging your phone for a long time does not damage your battery. Electrical Engineer, Timothy Awany at Uganda Christian University (UCU) explains why.
When you have your phone plugged in, what everyone calls a “charger” is just a power supply, like a battery or other DC adapter.
The charger is part of the phone’s power management circuitry which in most cases will first use that to power the phone. This is because a battery can’t be simultaneously charged and discharged.
So assuming the phone doesn’t suck up all the power, the phone’s charging circuit will start charging the battery.
The charging process keeps going until the battery is full. At that point, the charging circuit simply stops charging the battery. Your phone continues to run from the power input. If there are any power surges that can’t be handled by your wall wart, most phones combine the power from the input and the battery.
The battery charge controller (which is in the phone, not the “charger”) should stop charging the battery when it reaches its state of maximum charge without damaging the battery no matter how long the phone remains connected to the charger.
It is advisable to charge your phone at night before you go to bed. In the morning, you will be ready with a phone that will last you all day.