Arduino+XBee how to monitor the power supply

Hi, I am using Arduino UNO and XBee to build up a network. It is powered by battery. So I need to monitor the power value. if I use the INTERNAL ref,for example 5V.but when the battery's voltage descend. dose it keep steady with 5V? give me some help.thanks.

I usually just use a voltage divider to monitor car battery voltage, dropping them from 15V to 5V range. What type of battery do you use? To save power, you may need a switching regulator and study some power schemes on Xbee maybe?

I do not have any plan now. switch regulator sounds good. I am going to use it. Now I just need to monitor the power on my arduino board. Should I connect the 5V to the Analog input directly. what about when the battery's voltage desend to4.5? Would the reference voltage INTERNAL desend as well?

OK, here is a diagram:

You can sense battery voltage up to 15V.