Cigarette lighter voltmeter device question

John_Gray

There are many of these devices available on Amazon, and no doubt elsewhere, where you insert the device into the cigarette lighter and turn on the ignition to show the "battery voltage" to one decimal place.  Starting the car shows the "alternator charging voltage", about 2V above the former value.

What puzzles me is that many of the writeups say that one should not leave the device inserted in the socket, but remove it after checking the voltage(s).  Can anyone tell me why, please?  In most instances, the cigarette lighter only becomes live when the ignition is on, and three LEDs are unlikely to draw more than a few mA of current.  Puzzled...

Carrot007

All these products manuals are written by the same copy/paste text in china and the creator for your particular one would have had no idea what it said.

Use common sense! It is reasonable to unplug such an item if the lighter socket is still live after the ignition is off. It it does it for you then don't worry. (I have such a device on my double socket and 4 usb adaptor, though mine also has a button on the plug that turns it off).

DiddyDavies

John_Gray said:

In most instances, the cigarette lighter only becomes live when the ignition is on, and three LEDs are unlikely to draw more than a few mA of current.  Puzzled...

Many vehicles do have the socket powered all of the time and whilst it's true that LEDs only draw a very small current, this is only true if the device is operating correctly and as many of these will have been made for pennies using the cheapest possible components, it makes sense to unplug it when not required just in case it does fail when the vehicle is unattended.

AdrianC

Indeed - I suspect it's two-fold. Not drawing unnecessary current, and reducing the risk of a thermal failure going unnoticed.

Norman_Castle

John_Gray said:

There are many of these devices available on Amazon, and no doubt elsewhere, where you insert the device into the cigarette lighter and turn on the ignition to show the "battery voltage" to one decimal place. 

My previous one underead by 0.2V, I suspect its replacement is the same. They give a vague idea of the voltage, at least they underead by a consistent amount. For £1.29 they're ok. Mine's a combined usb charger and voltmeter so is clearly expected to be left in place while charging.

forgotmyname

If you leave it in a socket thats live 24/7 then it uses a small amount of power. Cheap ones may get warm.

If the socket turns off soon after parking then no issues at all.  If you do short journeys its ideal as an

early warning to charge the battery.

John_Gray

Interesting!  Effectively the manufacturers of these devices are implying that they are too dangerous to be left connected for long periods or when there's nobody in the car to 'supervise' them. 

Norman_Castle: my cigarette-lighter voltmeter also under-reads by 0.2V compared with a standard multimeter.  This must be a 'feature'...!

AdrianC

John_Gray said:

my cigarette-lighter voltmeter also under-reads by 0.2V compared with a standard multimeter.  This must be a 'feature'...!

Are you checking the voltage in the cig socket with the multimeter?

Because, if not, perhaps both are equally accurate, but there's a 0.2v voltage drop between the cig socket and wherever you're checking with the multimeter - presumably, the battery terminals?

forgotmyname

Yep a voltage drop of 0.2 is nothing to be worried about. You will learn whats normal and when its lower than that.

The wires inside the unit are probably super thin also. Less than perfect connection between the plug ans socket

it all adds up.

Mutton_Geoff

Just because something reads to n number of decimal places doesn't mean that is it's accuracy. A cheap tuppenny item from China to be taken with a pinch of salt. Even an expensive multimeter like a £150 Fluke will only be as good as the last time it was calibrated.