Here is the situation, I'm using a 5.7V battery to energize the arduino, I want to know the battery level using the ADC, if the supply input voltage is lower than 3.3V, the internal ADC reference voltage also decreases or it will be 3.3V? I mean if the power supply intput is 4.2V the internal ADC Reference voltage is 4.2V??
Default Aref is the MCU's supply.
If you measure the battery against the battery voltage, you will always get the same A/D value.
One way is to drop the battery voltage down to ~1volt with a voltage divider, and measure with the internal 1.1volt Aref enabled in the code.
See analogReference() - Arduino Reference
Another way is to use a software trick.
Google "Arduino Secret Voltmeter".
Leo..