Battery Voltage Warning lights - Idiot Lights
Here was a low cost project and was also easy to install.  It's a simple warning light to let you know if your battery voltage is low, or okay.  The little board that I found at Electronics Parts Outlet (EPO) cost me a whole buck! ($1.00) and took about 5 minuts to install.
The board has 3 LEDs on it, Red, Yellow, and Green.   Red, for low battery voltage, yellow for weak, and green normal.  From time to time the yellow LED may light up for a few seconds, or just glow a little when the lights or some heavy load is on. The red light will light up is the alternator is not working.  This helps to get you attention to the gauge should you have alternator problems. 
Installation was easy, I removed the four screws for the gauges, drilled 3 holes for the LEDs. Installed the board in place with the LEDs sticking out, and wired direct to the back of the voltage meter. Mounted the panel back into place, and that was it.  Quick and easy.
Click on photo for large image