Once upon a time, William Herrin <bill@herrin.us> said:
Ordinary ionization-based smoke detectors use a 10-year lithium battery, which is about the same lifespan as the americium-based detector circuit as it begins to decay into neptunium.
Also, some detectors are wired to household 120VAC service, so the battery is a backup, not primary, power source. I think this is required in modern residential building codes. My house was built 30 years ago and has this. I think larger homes even connect all the detectors together (so one detector going off can trigger all to alarm). And for typical 9V replaceable battery models, the "change the battery twice a year" bit is not based on the actual load, but just trying to get people to think about it (and maybe then getting it changed once a year, which is perfectly fine and maybe even still more often than needed). -- Chris Adams <cma@cmadams.net>