Battery issue

[matthg40]

Evening all I'm scratching my head here hence why I'm after advice.i put a new battery on my g40 3 days ago,went to start it this morning and it's completely dead.im sure nothings draing it as all that's connected to the battery is my alarm and gauges I don't even have a stereo.so I'm really scratching my head here the alternator seems ok and the supercharger belt is Nice and tight so everything is charging.