Lithium-ion batteries are not charging in a linear manner. When a user connects their mobile phone to the charger the battery charges from 2 volt to a peak voltage of 4.2 volt in two phases.
In the first phase of charging (0% to 50%), the highest peak current and voltage are drawn to the battery and it remains constant throughout this phase. Thus, most of the fast charging technologies are most effective when your battery charge level is less than 50%. After 50%, the output current of the fast charger begins to fall.
The second phase begins when the battery has received most of the charge. The charge controller (power IC) decreases the voltage and current drawn which prevents the phone from overheating and ensuring the smartphone remains safe. Because of this in most smartphones, support fast charging, after 80% charging become slow. For the amount of voltage and current to be passed, the charge controller circuit is used inside the phone. In most smartphones temperature sensors are also present to show the health status of the battery.
The basic USB charger sends 5V and 500mA to 1000mA at the output which is just 2.5 watts to 5 watts. Whereas fast chargers supply more than 10 watts (that is 5 volt and 2 ampere output). Every fast charger uses a common concept and that is supplying more power.
But does more voltage harm your mobile phone battery? For this, fast chargers use a buck converter. It is also known as a switched-mode step-down power supply which lowers the output voltage and increases the output current. More value of the current rapidly fills up your phone battery with charges.