Professional Grade State of Battery Monitor
This will mount close to your battery and will measure how much current goes in and out of your battery, and will tell you the state of charge regardless of the voltage. This system costs a good deal more than the monitor above because it gives you 100% accurate readings at anytime throughout the day. During the day, you will have what is called a "Surface charge" that will obscure your voltage readings (which will make it seem as if your batteries are full). If you use the a basic voltage monitor, (like the one above), the best time to see the state of charge of your batteries is in the morning before the sun rises.