Confusing question: How does our cellphone knows the percentage of battery left???

A typical cell phone battery is the lithium-ion type. It is fully charged when the battery open circuit voltage is around 4 volts. As energy is used the voltage drops off towards around 3.5 volts. This 0.5 volt change corresponds to going from fully charged 100% to around 20%. Somewhere just below 3.5 volts the unit will reach “low battery” and turn off automatically. The state-of-charge monitor circuit of the cell phone simply measures the battery voltage and indicates to the user by means of a display of “percent charge remaining”, often with a bar graph.

- Physics Forums

Thanks Vivek, asked many people and searched the web, couldn't find a proper, sourced answer. Your's sounds correct.