In sci.electronics.repair Danny D. wrote:
On Sat, 01 Feb 2014 05:17:38 -0600, Capt Rick wrote:

No need to sacrifice a USB cable. Measure the current going into the
cigarette lighter adapter.

I can just hook an ammeter across the fuse for the cigarette lighter
socket, which will give me the current being consumed with, and
without the phone being charged!

Check the current across the lighter fuse in three conditions:

1. Nothing plugged in to the lighter socket.

2. Charger plugged in to the lighter socket, but phone not plugged in
to the charger.

3. Charger plugged in to the lighter socket and phone plugged in to
the charger.

1 is because, on some cars, the "lighter" fuse also supplies a small
amount of current for other things, like keeping time in a clock, or
keeping the station memory in the radio - stuff like that. It might not
be weird to measure a few mA on that circuit even with nothing plugged
in to the cigarette lighter.

In newer cars, the trend is to have more fuses for individual circuits,
so this may not happen in your car. If your cigarette lighter switches
on and off with the ignition (a lot of Japanese cars do this), then this
may not happen in your car either.

Matt Roberds