it has current in it all the time... similar to your sockets at home. they have power but are not using it until an item is plugged in. you do not have to worry about draining the battery there is a meter built in and it cuts off power to accessories once the level dips below a certain threshold. on pre-face lift Q7 you could enable a battery meter in the MMI and if i recall correctly once it dropped down to 75% life it would shut off radio, mmi, lights etc. until you restarted the vehicle.
(a bit off topic but does anyone on here know if the battery level meter may be enabled on mmi-3g+ units? i tried a while back but did not have any success: http://forums.audiworld.com/q7-discu...mi-3g-2845903/