I've had a relay'd horn mounted on my bike for a couple months now, and there's no battery drain with it. I'm not a hugely experienced electrician but my understanding is that a relay basically takes a weaker electrical signal that's coming from the stock horn wire and uses that power to close a connection between the wire going directly to the battery and the new horn. So if the switch isn't being powered, there is no connection = no current draw = no battery drain.
Ear shaved, re-jetted with 142/42 & 2 needle shims.
New modified front end with Maier GP fairing, half-ape handlebars, supersport grips and bar-end mirrors.
Stock seat, cut and modified to remove the passenger backrest.
Factory-Spec Sportster shocks.
Self-fabricated rider's backrest.
Engine guards and highway pegs.
Custom-made straight pipes, fiberglass-wrapped exhaust.