Prevent Mobile over charging?
"I know there is an IC or circuit inside my phone that will disconnect the charger once the battery is fully charged"


Yes. Which is why you're not getting the results you expect.

Before USB, some chargers were directly connected to the battery and the phone lacked intelligent battery management. You would see voltages in the 3-4.2V range.

After USB, the "charger" is just a power supply and will always supply 5V. Charge control and end-of-charge should be managed by an IC inside the phone. It should theoretically be safe to leave it plugged in forever.

Overheating is bad for electronics, but they should not be overheating in normal operation. However, your charger is 5.5V rather than 5V, and this may be causing the problem. Counterfiet chargers can have the wrong voltage.

Reccomendations about what level of charge to keep batteries at are for long term storage (about 50% or slightly lower). Life may be improved by not charging beyond 80%. But, and this is very important, the state of charge displayed on the phone may not be accurate and may already account for this.