Ok, here is one concept.
You have an advantage here, ADC reading, that you can actually measure the voltage; it does not have to go off as you can continuously check the voltage level.
Since you are using a battery, power-off concept may not be a good (or let say precise) solution for you. But if you had a non battery operation with this circuit, this is the way to go.
Thus, reading (monitoring) the battery voltage at AN0 in your case is much more accurate.
Try and see....
<img src="http://img6.picsplace.to/img6/23/poweroff.gif" alt="Image Hosting by PicsPlace.to" >
Edit: I will post another circuit to read the voltage level with AN0.
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