This is a colorful, useful and educative project from Erlich Industrial Development, Corp. You will learn using Op-Amps as comparators and also LED bar graph analog display.
Voltage Monitoring is an Electronics board that allows you to connect 5 to 14VDC on one side of the board and get it corresponding LEDs light on the other side. Every Car, tractor or boat should have one.
The boards will allow you to easily monitor your 12V battery while powering from the same battery. The Terminal Block TB connector is included and makes it easy to use.
The LM324 is the Heart or Intelligence in this Circuit. There is a voltage divider and zener too. They have a kit and pcb to buy and build .
A battery of a car can drive enormous amount of current, anything that is High-Voltage, High-Current or High-Frequency or a combination of these; are known to be high energy circuits. So as a precaution when you wire the positive to the car battery use an automobile fuse with holder like the one used outside the stereo player. The fuse has to be outside!
You could also use a fusible resistor in a metal box and keep it outside, the 12V positive has to go thru this. Any car/boat project needs this. A short circuit on PCB or Wire will blow fuse and everybody is safe.