I'm sure this is a really simple answer but I'm completely forgetting it as I haven't worked on my Arduino in a month. What would the code be if I wanted one of my analog inputs read a voltage coming in. Not from a sensor or anything, just a voltage. It's going to be the voltage from my power supply so my robot will alert me when it's running out of battery. Any help would be great!
Thanks! 
Here is the key lines of code that you will need:
temp = analogRead(in_pin);
val =(float)temp * .0049 ;// val needs to be a float
There is 1023 steps when you analog read, every step is .0049 volts.
If your power supply will give more then 5 volts, you should divide the voltage with resistors(Voltage divider - Wikipedia).
If you divide your voltage in 3 or 2, you should multiply val by 3 or 2.
Hope I helped.