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I have two USB car plugs (you plug them into the cigarette plug and connect a USB cable to them) which also have a digital voltmeter.

I noticed they both massively underestimate the voltage. Is this normal?

E.g if (with the car switched off and the battery having been at rest for 24 hours) a voltmeter connected to the battery measures 12.4V, these two would measure 12.0 ; that's the difference between a 3/4 full battery and an almost flat one.

Is this just because both are cheap Chinese imports, or is there a deeper reason? E.g. since they can only measure once I turn on the car (I mean, twisting the key without turning on the alternator and the engine), does that mean the reading is lower because turning on the car draws so much current? If that's the case, these voltmeters aren't really much use.

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    I have found these to be inaccurate and worthless, best to read battery voltage at the battery for accuracy. – Moab Apr 26 at 14:39
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    Use a quality voltmeter, those things are best used as voltage is present or not. – Solar Mike Apr 26 at 14:43
  • Thank you both. Do you know if the reading is inaccurate because those thingies are too cheap. or if there is some other reason whereby reading the voltage from the cigarette plug after turning on the car will always be an underestimate? – Pythonista anonymous Apr 26 at 14:44
  • Do a cross-test: Use the voltmeter to measure the voltage at the cigarette socket, use the USB plug to measure at the battery. I guess both reasons are true. – Agent_L Apr 27 at 15:25
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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:

  1. It is measured by a precision instrument
  2. You have an accurate idea of the temperature of the battery
  3. 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.

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  • +1 Cigarette socket voltmeter is a gadget with no real use. It's fun to watch and can teach you how widely the voltage jumps, but gives no information about the car whatsoever. – Agent_L Apr 27 at 15:31
  • So if I get in the car, turn the key without turning the car on, read the voltage on the voltmeter then, without switching anything off, I check it with the multimeter, then I can know if the reading is accurate, right? – Pythonista anonymous Apr 27 at 15:39
  • Also, if turning on the dashboard the lights etc causes a drop in voltage, does this also mean that the voltage display embedded on many modern dashboards will be an underestimate, for the very same reason? – Pythonista anonymous Apr 27 at 15:40
  • Well to test whether the reading is accurate, you could try the gadget, and then measure the voltage with a Fluke multimeter, AT THE BATTERY TERMINALS (not at the cigarette ligher socket), and compare those two, you will know whether the gadget is useful. I think the voltage display on many modern dashboards uses a separate measurement circuitry, at the battery. So even though the dashboard has a measurement display inside the car, it probably uses a separate wire going to the battery, with no voltage drop. – juhist Apr 28 at 9:58

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