Arduino ADC values change after USB disconnect
Read the Atmel Datasheet for the '328

Section 28.2 cover the A/D
The inputs look like this:



By default on the Arduino Uno the ref voltage is taken from AVCC which is the 5 V VCC used by the MCU.

The Arduino schematic is here.

You can program the unit to not use AVCC/1.1ref and feed in a more stable reference ....or fix your supply voltage.

You could also switch to the 1.1 V reference and change your input divider.