The Smoke Detector Paradox

There are 24 hours in a day. The number of minutes in the day is equally divided among the 24 hours. However, despite this equal division of time, the battery in any given smoke detector will fail to the point where said detector begins emitting its warning chirp only during the nighttime hours, with a higher probability of occurrence during the "wee hours" between 2am and 4:30am — late enough to be deep asleep, too late to get back to sleep for any refreshing length of time, yet way too early to even consider just getting up for the day.

It happened again this weekend. At 4am Sunday morning, I hear my wife scolding my dog for climbing up on the comfy chair in the bedroom. What got him all riled up?


Oh, the smoke alarm. I get up and find the stepstool while my wife goes to the basement to retrieve a 9V battery. Replace the battery, put the smoke detector back up.


Wrong one. It wasn't the one that was installed with the house, but the one installed with the alarm system. The two are side-by-side, so it was an easy mistake — although if I had been more awake, I would've remembered that the alarm system had in fact been warning me that the fire alarm was reporting a low battery for a couple days. Oops.

Climb back up, take that smoke detector down. But it doesn't take a 9V battery; it takes two camera-style "123" batteries — not the kind you'd have sitting on-hand for an emergency change in the middle of the night.

So, I remove the batteries to at least silence the chirping — which immediately sets off the house alarm. My wife runs downstairs to deactivate the panel before the siren starts. Moments later (fortunately, before we are able to get back to sleep), we get a call from the alarm company to confirm the situation.

It took me 20 minutes to get back to sleep. I was lucky. My wife was up for over an hour.

I've complained about this paradox before, and the response I usually get is unsympathetic. "Well, you're supposed to change the batteries in all your smoke detectors twice a year anyway."

Except that would be overkill. Our smoke detectors don't run on batteries; they're connected to the house wiring, and the battery is used as a backup. As a result, the batteries often last a couple years or more. This one is a different case, as it was installed later and does run on batteries, but not your typical 9V, die-in-half-a-year variety. And at $10 a battery (and this thing takes two of them), you can bet I want to use them as long as they last rather than replace them on a premature schedule.

If I ever make a smoke detector, I will ensure that it obeys the following rules for when the battery is low:

  • Do not begin chirping if it is dark.
  • Wait until there has been light for at least two hours before chirping. (Sun comes up a lot earlier than I do in the summer.)
  • Once chirping has started, do not stop until battery is changed. (Annoying, but I'll concede for safety reasons not allowing someone to just deal with it during the day until they can sleep at night; what if they were out all day?)
  • If there has been no light for 10-12 hours, begin chirping. (In case it's not in range of visible light. Yes, there's a chance this could still go off at night.)

At most, this would delay the warning for half a day, which, considering for how long I've heard these things chirp, isn't that long. A little extra cost for the light sensor. Battery drain shouldn't be too bad, as it only has to sample light every half hour or hour or so, and only when it's about to reach low battery stage, and then only for the first half day at most. (And if, like mine, it's on house wiring anyway, that point's fairly moot.)

Perhaps I should patent this idea...

No comments: