What is a Car Battery Monitor Used for?
A car battery monitor is an electronic device that is designed to show at a glance, the state of charge of your car’s battery. Most of these can be dash mounted and have a series of LED’s in three colors; red, yellow and green. Green indicates that your car’s battery has a full charge and a full 13.6 Volts available. Yellow indicates that the charge contained by your battery has started to fall off and your battery should be checked. Red is the final color, and indicates that your car’s battery most likely doesn’t have enough of a charge to start the vehicle.
For more info, see How to Charge a Sealed Car Battery