~~Where I live, smoke detectors come with a guarantee that the (9v) battery will keep them operational for at least 10 years.~~
~~When the battery is low on charge, they start beeping in regular intervals and from when the beeping starts it still takes a year or two for them to run out. Which you will surely notice at some point when you're at home.~~
~~So I don't really understand why someone would need a notification.~~
~~I guess a notification could be nice if you have a hearing deficiency, but instead you could make a calender apointment to replace them in 8 years just to be safe.~~
~~The alternative is getting smart smoke detectors that are compatible with home assistant and that know their own battery level.~~
Edit: this last bit still applies and is one way to do it
Then you'd need to selfhost home assistant on some always on device. And buy compatible smoke detectors.
https://www.home-assistant.io/
https://community.home-assistant.io/t/smoke-sensor-with-battery-status-in-ha/357762