I assume you'll want to measure voltage higher than 5v? The light would go on at 4.5-5v to signal a low battery?
It is possible to measure voltages higher than Vcc using a voltage divider. See Experiment #2 on this page:
For a more accurate measurement, don't assume a resistor labeled 1k is exactly 1k. Measure your resistors with a quality ohmmeter to determine their actual values and plug those into your program.