Gutters, furnace filters, duct cleaning, dryer vents, detector batteries etc etc.

Still, plan on putting cash into the annual inspection and definitely plan on $20 for lightbulbs and smoke detector batteries.

I changed to Schlage electronic keypad (great for nicer areas and vacation rental type houses as you can change code easily and instantly and give a code to your cleaners) but battery had to be replaced surprisingly early (about 1 year - supposed to last 3) Your application dictates which lock is best.

Batteries and battery acids are also common, dry cells and lead acid for larger models.

This video showed a property in good condition but with 3 smoke detectors which the tenant removed in lieu of replacing the batteries.

I took this weekend to recharge my batteries.

I have also done the cheap battery powered push button lights.

something that helped me was to get a great PM on your side before you pull the trigger (hence promise them your future business), they will keep you out of the wrong side of town, give you realistic area rents and tell you what's trending, meaning what are young families looking for etc...then after everything is in place go the extra mile and spend time locating a local handy man you can pay yourself independently, typically an older gentleman who works for himself, make sure he's not affiliated with your PM, he'll be the guy you send to look at things to make sure your PM is keeping a tight ship, essentially another set of eyes, who can look at the bigger ticket works orders before you order them...so just tell your PM you have a friend who you like to send over to see the bigger issues...for example: i can tell you that i've avoided $4k HVAC replacement orders several times, given some AC people are dishonest...an issue arose at a house they sent the AC people and they wanted to replace ...but I call my guy before doing anything, so my guy goes in and identifies and even fixes a much simpler issue, in 2 cases replacing the Capacitor ...a $30 battery pack that fits onto the outside Unit...that's just one example...cost me $100 all told, these issues will kill your cash flow, and your PM may simply not have time or bandwidth to double check everything etc...

As I understand the material on Bigger Pockets, you charge rent, save half for reserves/repairs, then use the other half to pay the mortgage, and what's left over from that second half is your cash flow.  

They use a battery for backup, but if it beeps and they remove the battery guess what happens?