The reference voltage of analog ports are supply voltage
If supply voltage is lower also reference is lower. So you always measure 100%
You can set analog reference to 1.1V (internal reference) and increase the measurement voltage with voltage divider to max 1.1V (CAUTION! May damage your controller if you dont know what you do).
Now you should measure the power supply voltage.
If you set the analog reference to 1.1V this is for all analog ports (5V sensors don’t work)