+1 to the person above who mentioned recharging the batteries after an event.

Just be sure you change the batteries annually.

currently needing to upgrade a new property of mine and am stuck trying to figure out which would be my best option to go with: Combination CO2 & Smoke alarms that are 10yr sealed or just plain jane battery operated.

They're just the 'payment module' that goes on an otherwise industry standard washer. 

I tell them to change the batteries when i send them the lease renewal or else theyll get locked out. seems to work well so far.

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If you choose the self study route with no intention of taking the exam, start by looking at the core modules of the agent courses, and start following that trail.

Could be as small as a dead battery in a smoke detector in the middle of the night (no guest wants to hear beeping while they are trying to sleep), guests can get rowdy and disturb neighbors, I have even had guests that insisted upon having someone meet them face to face when they arrive.

fresh batteries in all.  

I don’t believe the digital wall mounted controller is operated by batteries.