http://www.uoguelph.ca/~antoon/gadgets/shunts/shunts.html
This can be used with proper scaling to measure current over time. You can read the voltage, properly scaled with another analog pin.
If you know that and you told the arduino when the battery was charged you can get an idea of how much power is left. If you don't trust your battery maker (HAHAHAHA) you would use the arduino to measure how much current is used until voltage falls below a certain level and to record that for you when you ask for it. Using those figures you can set up the values you need for your announcements.