12v usb socket advice reqd
 

Another question I can't find the answer to searching the forum. Does a dual 12v socket/5v USB charger wired direct into the battery draw current when nothing is connected. All I want to do is run an sat nav occasionally and electric air pump in case of a puncture. The wiring has a 15a fuse inline for the air pump should I need to use it.

My logic would say no, but then it should be without a LED light indicating power use

As above, yes if it has an LED, no if it does not, I fitted a relay (powered by headlight dipped beam) so my accessories all turn off with ignition, but are still powered from battery.

These USB-ports always draw empty my battery even if nothing is connected or if it doesn't have a LED. I think it's because those USB ports convert 12v into 5v. So there must be some kind of a converter in it that draws current.

Thanks for the replies, I have read what Arjen says that there is some voltage loss transforming voltage down from 12v to 5v but was unsure and wanted it confirmed. I will keep a track of the voltage readings and see if I can detect any loss.

Just to confirm all USB sockets consume current even when nothing is plugged in. If not isolated or ignition-switched the step-down transformer in them can consume up to 50-60 mA. This is enough to make starting after about 80 hours (3-4 days) challenging and completely flatten a 9Ah battery in 180 hours (a week).

USB sockets must be switched/isolated when not in use.

Many thanks for that definitive answer.