I had added some code to an existing program where it would read the battery voltage and display it on a LCD. When my UNO is plugged into the usb the display shows the "Battery Power...." code. When the UNO is on external power, there is a long blank pause where the battery display should show, then runs the rest of the proggie.
Both cases the LCD gets power from the UNO.
The external power supply is a 12volt battery pack through a filtered power supply.
Battery level ref is from a voltage divider from the 12volt battery pack.
This is the only time I have ever noticed this phenomenon.
Any Ideas?