I have a ‘friend’ who had left his master on for days and completely wiped out the PC680 battery. Twice. He has a different procedure for shut-down, now.
And the Odyssey is a deep cycle (that probably means different things to different people, not sure it means killed dead) battery.
Here here are a couple Odyssey docs for year reading pleasure:
http://www.odysseybattery.com/documents/ProceduretorecoverdeeplydischargedODYSSEY.pdf
http://www.odysseybattery.com/documents/US-ODY-TM-002_1214.pdf
The first time was at AirVenture. I left, I mean, my ‘friend’, left the master on after turning it off after being reminded by the nice folks working ground crew. Didn’t see it until 4 days later when we were ready to leave. Took it to the emergency repair shop, put it on a charger for a few hours, got enough volts into it to to barely crank the engine over, but the voltage jumped up from the alternator, and flew it home. (I had stopped by a couple vendors in the hangars. Plan B was a new battery.) The voltage grew from 12-something to 14.4 by time we were back to Omaha. BUT-the battery never really recovered. I kept using it another 6 or 8 months before replacing it. You could flip on the master and see the voltage drop over the course of less than 30 seconds. It started at 12.7 or so and would slowly drop to 12 or less. During start-up, voltage would head south of 10 volts no matter the ambient temp. When you’d crank, the first blade would stop for an instant before going though. Always started, though. Tried different chargers, but the battery was spent. Never had issues with voltage in the air (I have only 5 or 6 amp load). But it was toast. Finally decide enough was enough. Replaced the battery and was amazed at the new-found speed of the starter!!!!
The same ‘friend’ left his master on after a trip (after the previous event. He’s a slow learner…..). Luckily it was in his hangar at home when it happened. I, I mean my ‘friend,’ went to the local giant auto parts store and bought a 400A lithium-ion battery booster/starter. Hooked it up. The voltage went up to 10-ish volts (if recollection is correct) in like 15 minutes. Left that thing on for an hour maybe, trying to hit it with a lot of volts/amps. NO idea the battery got hot or not. Never removed it from the firewall or pulled the cowl. Put the trickle charger on over night. Went out the next day and it was fully charged. Now when I head out there, the voltage is 12.8V before start-up and haven’t really paid attention to what the voltage sags to, but its like the battery was never drained. And, no, I don’t put a maintainer on it.
So…. depends on whether or not you want to mess with it and what's an acceptable level of risk for you. They
are a deep cycle battery. With all that being said, I’m a VFR only guy. My plane doesn't 'need' electrical accessories (no electrical only fuel pumps, ignition requiring external power, etc) to typically fly. But it does make it a lot nicer! I might have a different opinion if I was in the clouds. As everyone on here says- YMMV.