An Alabama client of mine had this experience, but I think it is relevant nationwide. He had smoke detectors in all rental houses. It was the tenant's responsibility to test the batteries periodically and replace them.  In at least one house, a tenant of many years had not done that. There was a fire and the house burned to the ground. The insurance company denied coverage because the smoke detector was not operational at the time of the fire. My brother-in-law, a fire marshal, told me that even if the smoke detector is a melted lump of plastic and metal, they can easily determine if it was operational at the time of fire.  He said if operational, they will find evidence of soot inside the detector. No soot, not operational, no insurance coverage.

The lesson--the landlord should check all smoke detectors at least annually, and replace all batteries at that time, and invoice the tenant for the expense.