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Why do smoke detector batteries always fail at 2:30 AM?
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Because 4:30 would be more painful.
"If we don't change direction, we'll end up where we're going"
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Well ... it's actually physics, not design.
Battery output varies with temperature, and most houses are coolest between 02:00 and 05:00 (for obvious reasons). As they cool, the battery can't put out as much as it can during the warmer day, and the smoke detector picks this up as an imminent battery failure and starts the "replace me" chirp.
Or in our case it talks to you, which at 02:30 is even more disconcerting as it wakes you up and then shuts up so you hear something and don't know what it was ...
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Never throw anything away, Griff
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
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Don't you hate geeky correct answers to rhetorical questions?
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 Sometimes.
But ... I really like it when what looks like a random event that apocryphally happend 9 times out of 10 turns out to be a real situation that occurs because of the design of all such devices forces it to. 
Battery operated smoke detectors have to monitor the battery because if they didn't they would fail unnoticed and risk lives. But using a temperature sensor in concert with the battery output wouldn't help because if it relied on the temperature change affecting the battery, it would fail by day when it's less likely to be noticed, and not work at night when more domestic fire happen. And (of course) fires alter the temperature).
So it's a serendipitous safety feature, which I love the whole idea of!
Sent from my Amstrad PC 1640
Never throw anything away, Griff
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
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Mine died when I wanted to sell my house ...
I come home after multiple showings and 3 of them were chirping!
Why then? Why did 3 start at the same time? If multiples fail at the same time, why didn't all of them fail at the same time?
Very odd coincidence.
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Probably because you buy batteries in multipacks, like I do. Smoke alarms don't use a lot of power, so it's quite likely that all the batteries will fail at the same time.
Generally, if one of mine starts yelling at me, I'll meter the batteries in all of 'em and end up changing them all anyway to avoid the "02:30 wake up call" in a couple of weeks time. Which makes it more likely they will all start yelling at the same time.
Sent from my Amstrad PC 1640
Never throw anything away, Griff
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
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Pualee wrote: I come home after multiple showings and 3 of them were chirping!
Some are wired together so that if one goes off, they all go off. Unfortunately, it makes it impossible to figure out which one needs the battery replaced.
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Like others, when one battery fails I replace them all. I learned this lesson the first time it happened -- 3 smoke detectors failing within a week.
This wasn't as bad as when the sensor in a CO detector failed. We got the babies out and I opened all the windows. Took an hour to find the documentation that said either the battery was failing, the sensor was failing, or we were going to die.
Given that the battery tested good and we hadn't died, I was quite certain it was the sensor ...
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Most CO detectors have a built-in kill switch around 10 years because the sensors don't work anymore.
Always write the install date on them.
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englebart wrote: Most CO detectors have a built-in kill switch around 10 years because the sensors don't work anymore.
Always write the install date on them.
You may have mixed up smoke and CO detectors. Smoke detectors are supposed to last 10 years (just replaced mine and the warranty including backup battery is 10 years). According to several handyman sites I checked, CO detectors last 5 to 7 years -- although my story took place 20 years ago and we had a failure at 3 years. I don't know if the CO sensors last longer now or if I had a defective one.
Regardless of that, your advice to record when installation occurred is spot on!
Beyond that, I'm now in the habit of writing the purchase date and where purchased on the user manual for everything that comes with a user manual. Anything that doesn't have a manual gets recorded in a small notebook, which goes in the filing cabinet along with product documentation.
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Did the ghosts follow you to your new house, as well?
I wanna be a eunuchs developer! Pass me a bread knife!
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My guess is that because they are screaming "change my battery" Wherever you have them is coldest at night. So that's when the battery finally dips below allowable threshold.
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go for the zero power option: hang up some whistling kettles instead.
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Lopatir wrote: go for the zero power option: hang up some whistling kettles instead.
That will do for the heat, I think you might want to splurge a bit a get a canary as well for the smoke.
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Similarly, I used to say that a wireless mouse/keyboard's batteries always die an at inconvenient time.
Until I realized there's no such thing as "a convenient time" at which a battery could die.
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you could mod and use your wireless mouse on a phone charging pad.
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My mod was to get a wired mouse again.
Hasn't failed me yet.
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If you are in the UK they should be mains wired
We can’t stop here, this is bat country - Hunter S Thompson RIP
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I'm in Canada, and I'm not aware that I'm required to have smoke detectors at all in my residence, but I do get a break on my home-owners insurance for having them. I think maybe new builds are required to have wired detectors, but my house dates from the 60s, so it would require a retrofit in any case.
Are you saying that a 15th C cottage in the UK needs to be retrofitted with mains wired smoke detectors?
My battery operated smoke detectors have a sticker saying "replace in 2020", at which point I'm guessing the smoke-detecting ability has degraded beyond any usefulness. Assuming that wired-in detectors have the same issue, do you have a replaceable detection unit, or do you need to wire-in a new unit? Either way, will they start chirping in the middle of the night to let you know that the detector needs replacing?
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No, but if you have a rewire or anything that requires a certificate from an electrician they have to fit wired in smoke detectors
We can’t stop here, this is bat country - Hunter S Thompson RIP
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reading through the comments took me a moment to realise the nesting of the comments were seperate:
dandy72 wrote: My mod was to get a wired mouse again.
pkfox wrote: If you are in the UK they should be mains wired
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I'm not sure if wired-in is a universal requirement in the USA, but it is in my county.
Mine are wired in -- currently the backup batteries are typically warrantied for 10 years, after which the unit should be replaced.
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Here in Norway I have never seen anything but battery driven smoke detectors in private homes. And you are required to have at least one on each floor (in every housing unit if there is more than one).
The smoke detectors are required to warn you long time before they run out of power; I believe the requirement is a minimum of two weeks.
If you have more than one smoke detector, you are not required to interconnect them, but it is highly recommended. My previous set of detectors was made for wired interconnection, but before I got myself up to actually stretch the wires, the detectors were ready for replacement. My next (i.e. current) set of detectors don't need any wiring, they have wireless interconnection.
There was some discussion, maybe a year ago, after lots of media reported on a study showing that you should never keep the door to bedroom open while you sleep: If there is a raging fire in the corridor outside your bedroom, a closed door can hold it back from your bedroom for the few second it takes you to escape from the bedroom e.g. through the window. But the study failed to point out that if your home is properly equipped with interconnected smoke detectors, you would be awakened by a siren several minutes earlier, with a fair chance to stop the fire before it develops, and certainly before the raging flames reach your bedroom door. ... So I still sleep with my bedroom door open!
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