[SAI_Peregrinus]

Yep, and that's a big reason why combined Smoke + CO detectors are a bad idea. The smoke detector side doesn't wear out, the CO detector side does.

The beep patterns for "dead battery" and "detector worn out" are usually different. Generally they're printed somewhere on the unit, in a tiny font that's hard to see bleary-eyed at 3AM when it inevitably starts.

[tlavoie]

Smoke detectors usually have something like a 10-year usable life, and should have a sticker to that effect along with its manufacture date.

I've had a couple that when they aged out, went straight to full-on alarm beeping.

[SAI_Peregrinus]

True, this does depend on the detector type. Photoelectric detectors last pretty much indefinitely, but ionization smoke detectors do age. And you ideally want both, since they detect different sorts of combustion. Also there are smoke detectors you can buy with a 10-year life lithium primary cell, so no need to change batteries. These are perhaps a bit wasteful (the cell isn't replaceable and the rest of the detector could be fine) but also don't require throwing away lots of 9V batteries.

[tlavoie]

Oh, true enough. Or at least, by the time the battery gets low, the whole thing should be replaced anyway. As you say, depending on detector.