When my old battery powered smoke alarms used to get 'low' in power, they used to bleep at me.... and I would know to replace the batteries. Fair enough.
But when I had the house re-wired, they fitted mains powered smoke alarms..... which I assumed needed no battery.
I thought I might save a few bob.
However, three years on, one kept bleeping all last night, and kept us awake... so I switched the mains off and removed the battery. I was surprised to see a battery in there!
What is the point of a battery in these devices, if the mains is a constant supply of power?