Once the battery is fully charged and removed, use a voltmeter to measure the voltage at both ends of the battery. Since the voltmeter has high internal resistance, the current generated during measurement is almost negligible. The voltage measured at this time is called "terminal voltage" and the voltage obtained is the highest.
Once the battery is connected to the circuit, it will generate current through the external circuit. The product of the current and the internal resistance of the battery is the battery voltage drop. The voltage measured at this time is the voltage across the battery minus the battery voltage drop, so the voltage will be lower.
As picture shows: