In all the leases I am clear the batteries are the tenant's responsibility and make sure to tell them if it blinks red it's time to replace the battery. 

Blink is an Amazon company and they have wireless, battery operated cameras with motion detection at a very high quality and relatively low cost. 

What would you charge to diagnose dead batteries in a thermostat.....$75 trip charge?...

The Ring is nice because they have a battery operated one so thieves can't cut wires.The Nest is nice because the interface for reviewing/saving videos is easier to use in my opinion.We use both.Mike

I can only imagine how many calls you would get asking to be bring keys to let a tenant in because the battery is dead in a smart lock or some other issue (and I have smart locks on my personal home, just none of my rentals).

If you're using a keypad lock, what if the batteries die and they can't get it in? 

A third use is lead acid for batteries.

Any suggestions for an inexpensive preferably battery operated intercom/doorbell?  

The only thing to consider is the batteries need to be changed every three months and it requires internet access to operate.

Those batteries are not cheap and have short life-spans.