The problem with cigarette plug voltmeters are two-fold:
- They may not be precision instruments. Best to stick to a reputable multimeter brand such as Fluke. (I have found at least one cheap Mastech model is accurate enough, but I can't recommend cheap voltmeters in general.)
- You need to access the cigarette lighter socket to be able to use these plug voltmeters. In order to do so, you need to open the door of the car. If you open the door of a modern car, it will turn on all sorts of electrical apparatuses, causing voltage drop. They will not be turned immediately off when you close the door; there can be delay from minutes to tens of minutes. So, in practice to use the cigarette plug voltmeter, you need to enter the car, close the door, wait 10 minutes or more without touching any of the pedals (e.g. brake), and only then use the voltmeter. Then you can have an accurate reading, assuming the cheap plug voltmeter is accurate enough in the first place (hint: it probably isn't).
Me? I'm lazy, no time to wait for 10 minutes, so I measure the battery voltage at the battery terminals. There is no voltage drop even if you opened the driver's door recently causing some electrical systems to be activated.
Then, after using the voltmeter, you need to know the temperature of the car battery to interpret the result (lead-acid battery proper voltage varies as a function of temperature). An outdoor thermometer is good approach, but even better approach might be to measure the temperature of the battery itself via infrared thermometer. Or via a traditional thermometer if you have time to wait for the reading to stabilize.
The open-circuit voltage reading of a car battery is useful information only if:
- It is measured by a precision instrument
- You have an accurate idea of the temperature of the battery
- You have let the battery rest for a long enough time; there could be surface charge if you measure the open-circuit voltage immediately after driving the car
Oh, and if the cigarette lighter socket isn't of the "always on" type, needing you to turn on the ignition current, forget using it entirely! With ignition current on, there is lots of voltage drop.