With nothing plugged into the outlet it still drained the battery by virtue of it's existence? Is that right?
I ask because I'm looking to eventually install a 12 volt outlet on my Kioti, but I've never done it before.
The usb outlets will typically have a small transformer built into the plug to drop the voltage down to the usual 5v. This transformer can have a slight parasitic drain even if nothing is plugged in. On the other hand a cigarette lighter plug has no transformer and therefore no parasitic drain unless something is plugged into it.
I hope this helps!!