This compact battery and alternator tester is an absolute bargain and is a great little gadget to keep in your car. You simply plug it into your cigarette lighter, and the LED indicators reveal the status of your car battery and alternator voltage. For example, when running the alternator test a yellow light will indicate that the voltage is too low, a green light will highlight that voltage levels are normal, and a red light will showcase that they are too high. This is a quick and easy way to run a system test and check the overall health and status of your alternator and battery. Since this item is so compact (not to mention an absolute bargain) it is a handy device to keep in your glove compartment.