AA Undershield with battery monitor?

I have a project I need to run off batteries. I've built the AA Undershield (The Arduino AA Undershield : 6 Steps - Instructables) and it gives me the runtime I need (> 20 hours), so far so good.

I was thinking about connecting the +3v from the batteries to an analog port to measure battery life. I think this should work, but I don't know enough about hardware to make sure this isn't a bad idea.

Is there any reason I shouldn't do this?

Thanks in advance.

The Ground pin of the Arduino is connected to the negative terminal of the battery so you should be able to run positive side of the ~3V battery to an Arduino analog input without problem.

Thanks John!