Unless it’s a smoke detector. In that case when it is running low on battery - despite also being hardwired - it should definitely warn you about the low battery… at 3am. Always. At. 3am.
More annoyingly, they'll beep intermittently with no other indicator.
I have a house with ~15 fire alarms and when one is making an intermittent chirp every 30 seconds, it takes forever to figure out which one is low on battery. This could be solved with an LED that is lit when in a good state, and turns off when the battery is running low (or vice versa). For some reason I can't find a fire alarm with such a feature.
