Here in the US, there's a dirty little secret about smoke detectors. The common [cheap] ones are all ionizing detectors. And they ALL go bad in 5-7 years time and die as the ionization chambers clog with airborne dust particles.
So replacing the batteries is a nice concept
, but the new trend is to go to sealed units with a 10 year battery
, and just toss the whole thing when that expires because the whole detector is pretty certain to be dead. This is often cheaper than installing a new "9 volt transistor" battery
every year, and even if you buy cheap
batteries, those last well over a year each.
Dead detectors? Yeah, folks keep blaming the batteries. Meanwhile, the manufacturers and the fire associations have been failing to publicize the fact that the detectors themselves have a very short working life. And the ones that will last 'forever', the photocell ones, have other drawbacks such as taking at much longer to go off.