Project Questions about Programming and Electrical

[RyanN]

First off, a description of my project:
I started building a contraption that I call the "Weather Alert." It basically houses a 12V battery, microcontroller, switches, LEDs, Power Inverter, 12V outlet, 120V outlet, and integrated flood lights. My last version of this thing used a BS2 controller, but that proved to be very flaky and didn't have enough ports for me to use. Now this one is controlled by using the 2005 robot controller. I have the controller to run continuously in autonomous mode without the Operator Interface so all programming will be done int he autonomous code. Here are my questions:

Since I know nothing of programming, what variable is used to show the battery voltage, and if there is an equation to detect the exact voltage, can you please supply it?

Is there a way to output a frequency to a speaker to use for an alarm? This will have to be capable of producing many different types of sounds.

How would you charge the battery for this? I was thinking of just using a regular 20AMP Headlight relay to connect and disconnect the charger, but if I do this and run the battery completely down and it cannot connect the charger, how would I charge the battery? I can't have it setup on a normally closed circuit either because when I turn the Weather Alert system off, the charger would remain on, killing my battery(ies).

Some additional information:
My power switch is a DPDT Momentary switch. Both poles are used: one gives the system a "jumpstart" so it can come on (Switch gives power to the power relay, which provides power to the whole system), the other pole is used to send the kill signal to the robot controller.

I think I have a way to connect the charger by using some diodes and letting the RC only charge the battery for like 30 seconds if the battery if full or continue to charge it if it is dead.

If this all seems very confusing, it probably is... Just ask me a particular question and I'll answer it to the best of my knowledge.