Well.... I'm making one for the contest here.... but I'll be late due to exams....
Basically all you need is two resistors.... and some math...
Google "voltage divider wiki" and read for an instance....
Then get some reading to the
Thing is that I'm building a batt accumulator, with current sensors, serial to microcontroller interface
and such.... So I'll be late... but what you'll be needing is only the 50$ and two resistor!!!
And a buzzer to warn you that the battery is ending!!!