Question

what are the indicators of fully charged battery?

Answer 1

There are two easy ways to tell when a battery is fully charged:

While charging: Measure the current flowing into the battery. If the current is very low, (<0.05C) the battery is nearly charged.

This number can be lower depending on chemistry, but around this ballpark is good enough.

While Idle: Measure the terminal voltage, if it is equal to to around the max charging voltage, it should be fully charged.