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I have a USB micro charging adapter for my smartphone for my car. It is basically a cigar lighter plug that turns into a USB micro plug. When I don’t have my phone connected, and as such don’t want the adapter to draw any current, is it still safe to keep the adapter plugged in?

I’m wondering because most other AC adapters (i.e. those not for the car) usually keep drawing some current, even if no device is connected. Is that also the case for those car adapters? Should I rather unplug it whenever I don’t need it?

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    \$\begingroup\$ DON'T EVER LEAVE even your CIGARETTE LIGHTER in it... I burned a TRUCK UP one time, and I mean it BURNED UP to nothing but TRASH because when I used the lighter and pushed it back in, it did not pop out... we woke up about 3:30 a.m. to the fire department putting it out.. someone had driven by and seen it and called them. It was parked under the garage beside the house and the house had just caught on fire when they showed up. I have never again left a cigarette lighter in it at all much less anything else in there because it's better to be safe than sorry. We could have lost our lives! \$\endgroup\$ – Christy Mar 18 '17 at 14:17
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    \$\begingroup\$ While that’s generally good advice, note that a cigarette lighter has the job to actually produce enough heat to light up a cigarette (or other things :/). That’s very different to a charging adapter that will not produce these kind of heats. So unless it’s a very faulty device, it should not be able to burn up anything. And as I mentioned in a comment to another answer, my car actually turns off the power on the plug, so there is no risk in this case. \$\endgroup\$ – poke Mar 18 '17 at 14:38
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    \$\begingroup\$ Smoking is expensive. \$\endgroup\$ – Harper Oct 29 '17 at 13:21
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A good quality adapter will probably draw some current - but not much.
Strangely - a poor quality adapter MAY draw none.

A good quality adapter will use a switching regulator to step the voltage down - probably a "buck regulator" The quiescent (no load) current draw will vary depending on the design but I'd guesstimate it could be as low as 10's of microamps and would hopefully not be more than say 5 mA.

A load of 5 mA will take 200 hours or about 8 days to drain 1 Ah from the battery. That's about 2 to 5% of a typical car batteries maximum capacity. So even if you left that connected for a year it would probably take not more than about 1/2 of the battery's capacity. as you start a car far more frequently than that it should not be a problem.

A low quality adaptor may use a zener diode dropper - I've seen it done. This can draw no current at all when there is no load, but the output varies badly with load and it wastes more energy that it outputs.

Most adapters will be active switching regulator types.

You can easily test the quiescent current draw.
Using a 12V power supply or a car battery and a multimeter with mA ranges.
Set meter to low current range. (You may want to start on a higher current range to protect the meter from violence or stupidity. Operate adapter with no load powered by 12V and with meter is series with the battery leads so you can measure current.

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    \$\begingroup\$ If we want to just get the adapter consumption, shouldn't we do at least two measurements? One with no adapter installed and one with adapter installed. Today's cars might consume enough current to make an impact on the reading and on some cars, the ignition switch may need to be in on position to power the lighter port. \$\endgroup\$ – AndrejaKo Nov 4 '11 at 0:58
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Does your car switch the power to the lighter socket?

I know in my car, when I turn off the ignition, it disconnects power from the lighter socket.

If your car does as well, it would be fine to leave in for an indefinite period of time.

It's simple enough to test, simply plug your device into the USB charger, and see if it stops charging when you turn off the ignition.

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  • \$\begingroup\$ Apparently it disconnects the power from the socket. Thanks for reminding me of such a simple way to test ^^ \$\endgroup\$ – poke Nov 5 '11 at 15:18
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Yes, it draws some current when plugged in. Typically <5 mA.

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  • \$\begingroup\$ So don't keep it plugged in for more than 10 000 hours without charging the car battery. :-) (50 Ah / 5 mA = 10 000 h) \$\endgroup\$ – Oskar Skog Oct 29 '17 at 14:01

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