This is related to Is it okay to keep a USB charger plugged into the cigar lighter? - I have a 12v charger that is connected to a phone dock with an integrated micro USB connector.

The difference between this and the linked post is that there is an LED on the 12v plug that lights when power is supplied. Am I ok leaving this charger plugged in to the 12v socket when the car is parked? How much drain on the battery would the LED and empty dock cause? How long would it take to drain the battery enough to cause starting problems?

Edit: The 12v is powered on ignition off.


closed as off-topic by Nick Alexeev Feb 15 '16 at 17:32

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  • \$\begingroup\$ Your car doesn't disconnect the accessory port in full stop? \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 7 '13 at 11:28
  • \$\begingroup\$ @IgnacioVazquez-Abrams No, it remains connected. \$\endgroup\$ – SilverlightFox Oct 7 '13 at 11:38

How much drain on the battery would the LED and empty dock cause? How long would it take to drain the battery enough to cause starting problems?

Assuming the cigar lighter is powered when the ignition is off (which is not the case in any car I have owned)

A LED might draw 20 mA

A typical car battery might have a capacity of 60 Amp hours

A very rough calculation would say that the battery can sustain this current for 60 / 0.02 hours or 125 days.

In practice, there are probably lots of other loads on your car's battery. For example it probably has a remote control for unlocking the doors, the receiver for this has to be powered.

  • \$\begingroup\$ My current car (a Ford) and previous car (a VW) do power the 12v socket on ignition off. \$\endgroup\$ – SilverlightFox Oct 7 '13 at 11:39

It is not just the LED that is being powered. The rest of the charger is as well. And depending on the build quality of that charger, which parts they used, which parts they didn't use, etc, it can vary.

On the low end, a few mA for the LED itself, and a few µA for the switching regulator charging circuit (quiescent current). On the high end, 20~30mA for the LED, a few 10s of mA for an inefficient LDO, badly designed passive parts, you might be looking at 100mA or more.

AND THEN there is the phone dock. Is it just a empty plastic passthrough dock, or is it an active dock with features and stuff? If it's the former, it's basically just a cable. If it's the latter, it's a load on the charger, and that means an increase in current draw.

Only way to tell the actual draw is by using an ammeter or multimeter to test it.

As for affecting the battery, it highly depends on your battery's condition. If you do a lot of short 3 minute trips in one day, you quickly drain the battery. If its years old, it's capacity might be drained. There is no single answer. Consider that in my Camry, the map light is 200mA. On my old battery, leaving that on for 10 hours will make me need a boost. On the new battery, leaving it on for 14 hours didn't even make the battery strain at starting this morning.

In short, there is no one answer. Either use a switched cigarette outlet or chance that your battery might drain


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