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.

KORE Power, which was founded in 2018, said this would be the first lithium-ion battery manufacturing facility owned by a U.S. company.In addition to the 3,000 would-be KORE Power employees, the company estimated that the new plant would bring an additional 10,000 direct and indirect jobs to the selected region.

I most likely would get the electric battery powered ones that are now available. 

Check those smoke alarm batteries, clean the filter in the AC, and check all the windows to make sure you are getting a good seal....