[Auhydride]

exactly, I think you can show how much energy is left in the batts by changing the frequency or pattern (or both) of the beeper, maybe with a piezo, people learn these fast if the cues are well designed.
If you are going to use an alarm, you can just use a mosfet and cut the power to the aeg, it would be more useful than an alarm imho. What I would want would be a audible battery meter.
Of course it shouldn't beep all the time, it should have a way to trigger when you want.