I have a project that i would like to run off of external power through the dc jack and also have a backup battery connected to the Vin in case of a power outage. Being the case that the Uno has automatic power select, do any of you know the priority order of the source inputs? i need to know this because if the Vin takes priority over the dc jack, then my backup battery would just be used until it ran out and would render itself useless.
The arduino auto-voltage selection circuit switches between USB power or External power and if both are available it uses the external power. The DC power from the DC connector is the same as the Vin pin so there is no switching between DC connector and Vin.
So your idea of using a external battery wired to the Vin pin as a backup source for the normal power you will be providing to the DC connector will work. You should wire this battery voltage via a series diode to the Vin pin (cathode end wired to the Vin pin) and the battery voltage should be a little lower then the DC connector voltage in normal operation. The two diodes (one already wired on the board between the DC positive connector and the Vin pin, plus your new external series diode wired to the battery will act like a automatic voltage selector switch passing only the path that has the higher voltage, the other will just float until the other voltages goes away or is lower then the 'standby' battery voltage.
Both the external voltage and the battery voltage must be in the 7-12vdc range as recommended by the arduino specs.
That make sense?