This is a useful device to have in your car but you must understand it's limitations. In many cars, it will not be as accurate as taking a reading directly off your battery, which you can do with this device, if you buy this inexpensive additional adapter:Roadpro 12V Battery Clip-On and Cigarette Lighter AdapterThe reason for this is that in vehicles where you have to turn on the ignition to ACC in order to power the cigarette lighter port, once you do that, the vehicle's electronics are also powered and will draw power. So, you will see a lower reading on this device than if you take a reading directly from the battery with the key out of the ignition. In vehicles where the lighter port is always powered, this device can be more accurately used directly plugged in to that port.However, for taking a reading of your charging circuit with the engine running, this can be used in the lighter port in any vehicle.Note that you can do all of this with any household multimeter however the chances of you having that in the vehicle are slim. Most all multimeters also require a battery in them so even if you store one of those in your vehicle, the chances are high that when you need to use it, the internal battery will be dead.This Equus unit however requires no internal battery (at least I don't think it has one), so you can just throw it in your glovebox indefinitely.Car batteries typically last around 5 years so after about 3.5 years is when you may want to periodically use this device to check the health of your battery. Basically, anything under 12v after the vehicle has been sitting for at least 4 hours is a sign the battery should likely be replaced. Above 12v and the battery is probably fine as long as the vehicle starts up.When a friend or relative asks me to check their battery, the first question I ask them is "Does your cigarette lighter port work all the time, or just when you put the key in the ignition and turn on the car?". If they say it's always on, I know I can pretty adequately test their battery just through the lighter port. If however they say it only turns on when they put their car to ACC, I know I will have to have them open their hood so I can read the battery directly with the Equus.The other good thing about this is that you could leave it plugged in all the time for a constant monitoring of your electrical system voltage while driving and parked.For the price, this device is nice to have if you understand how to use it.If you want a more robust battery tester that can not only give you the voltage, but also the CCA rating (cold cranking amps), I recommend this:SOLAR BA7 100-1200 CCA Electronic Battery and System Tester