If it just started doing this, either the battery is going bad, or there is some circuit you don't know about that's draining juice.  You should be able to turn off everything just like normal, then disconnect one of the leads of the battery, and put a DC ammeter in between the removed lead, and the battery terminal.  That will let you read the actual current that's flowing through your compass, and anything else that might be turned on.  The lower tech method would be to just remove one lead of the battery when you leave the plane, and check the voltage when you return.  If it's still losing voltage that quickly, with nothing connected, it would have to be the battery.

How's your charging voltage?  If it's too high, you may be boiling off the liquid, and killing the battery.  Should be around 13.5-14.5 V with the engine running.

When you do replace the battery, remember the word- Odyssey.  VERY impressed with these batteries.  During the 3-4 month Rev-2 upgrade, I never charged my battery.  I also ran the starter and other electrical items along the way.  When it came time to actually crank the engine,  it was like the battery had a fresh charge.   Just amazing.

Cheers,

Rusty (no comment)