Note that there is a small chance you have degraded your chip as one of the
absolute maximum ratings limits the voltage difference between Vcc and AVcc
to 0.3V or similar. Typically for sensitive analog circuitry you would have
two identical voltage supplies connected, so that the digital noise is kept away from
the analog parts of the chip - such supplies would be generated using identical
regulators or the analog one would be LC filtered from the digital one.
The Arduino has no such subtlety and Vcc and AVcc are the same bus. Analog noise
isn't really a problem when battery powered, as a 10 bit ADC isn't too demanding.