Battery life in hardwired smoke detectors

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Old 12-08-07, 09:30 AM
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Battery life in hardwired smoke detectors

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Originally Posted by Rick Johnston View Post
...Plus, we're supposed to change our smoke detector batteries when the clocks spring forward and fall back.
I'd like to see some statistics on battery life & backup power consumption in hardwired smoke detectors. I routinely get over two years out of my 9V batteries in smoke detectors. I have 11 in my house, interconnected, and it would be nice to see some proof rather than "It's just a good idea."

How do we know this "good idea" did not originate with a battery trade group, or a firefighter with stock in a battery company?

I realize somebody out there will say "Proof? How about your dead child laying in bed?" but I'd rather see something based on science than emotion.

I won't deny that there may be some detectors out there that need new batteries more often, but I date all my batteries and nothing lasts less than two years. Multiply that by 100 million households and you see a lot of battery sales. And millions of still-usable batteries in the trash.

The money is better spent ensuring that people with old smokes get them replaced with hushable models so they aren't taking the batteries out every time they burn the toast.
 
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Old 12-08-07, 09:42 AM
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The literature will tell you tio change your batteries once per year. They generally tell you when you change the clocks. Since we change the clocks twice per year, some people will tell you to change them twice per year.

I am with you, and I change mine when they go bad. however, I always test them monthly. I have a reminder on my computer set and I get the kids involved. It doesn't take long to walk around the house and push the buttons.

The reason that people will tell you to change the batteries because MOST people don't test them and SOME people don't bother to change the batteries when they go bad. They don't have a battery and forget to get one or they are just too lazy.

The theory is, if yoiu change the batteries once per year, the batteries will never go bad.

Tragically, you read stories all the time about people dieing in house fires and read that the smoke detectors don;t have batteries in them.
 
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Old 12-08-07, 12:23 PM
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If you have SD powered by batteries only, it makes sense to change them on a schedule, since otherwise they may go completely dead without your notice (because the warning chirp will only sound for a limited time).

But scheduled changes of the batteries in hard-wired smoke detectors makes no sense to me. I think somebody forgot to update this advice when these dual-powered SD came on the market. When the battery goes low, the SD will chirp forever on the hardwired power, and it will be hard to forget to change the battery.

The only downside is the scenario in which the house power goes out, the batteries grow weak, chirp for a while, stop chirping, and the house catches fire, all before power is restored. I think if the power was out that long, then you ran out of money to pay your power bill.

Replace the batteries when they chirp. They will last for many years.

On a related topic, SD manufacturers only rate their units for a 10-year life. I'm not saying you necessarily need to replace them every 10 years, but if yours are 20 years old, I think you should replace them. I'd recommend replacing them all at the same time to ensure compatibility.
 
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Old 12-08-07, 03:00 PM
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For detectors that are not hard wired, every 6 months is recommended. With daylight savings time expanded, it doesn't make sense to change them with the time change anymore. For wired ones, I would replace them before they chirp in the middle of the night and wake you up. I think the one minute chirps are more of a hazard than a help; as you will disconnect the smoke detector out of desperation because you can't sleep and your dog is freaking out. Smoke alarms fail at the rate of about 3% per year, so in 10 years, 30% of your smoke detectors will not work and in 30 years, 100% will not work. This is why they recommend replacement every 10 years.
 
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Old 12-08-07, 04:10 PM
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Originally Posted by John Nelson View Post
SD manufacturers only rate their units for a 10-year life. I'm not saying you necessarily need to replace them every 10 years, but if yours are 20 years old, I think you should replace them. I'd recommend replacing them all at the same time to ensure compatibility.
Apparently I am not as contrary as I thought on the battery replacement. In almost ten years here, I think we've had one outage that lasted a half hour. The rest have been a couple of seconds to a minute or two. I think our phone service has been down longer.

My oldest detectors were installed in 2001. I added two more photoelectric and two CO/Smoke detectors in 2004. Next time out I want to do the voice-synthesized ones, and I am fantasizing about the wretched excess of fan and furnace shutdowns. Not to mention that fire sprinkler system, although the $80 per year insurance savings would pay me back in less than 125 years.
 
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Old 12-08-07, 04:41 PM
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Originally Posted by Strategery View Post
With daylight savings time expanded, it doesn't make sense to change them with the time change anymore.
I don't think 3-4 weeks is going to make any difference. For batteries anyway -- I won't even get into all the other problems that change has caused.
 
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Old 12-08-07, 07:44 PM
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3-4 weeks in the spring and fall though. Now there are about 8 months on daylight savings time and 4 months off.
 
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Old 12-08-07, 08:18 PM
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Good point Strategery, I didn't take that into account. At any rate, I don't if I've ever heard a recommendation to change your smoke detector batteries every 6 months. The DST thing was just a convenient time to get people to remember. The US Consumer Product Safety Commission recommends annually changing batteries.

Now why not, instead of the DST battery change, recommend changing them on Thanksgiving so your detectors are working when your Christmas tree burns your house down in 4.27 minutes flat?
 
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Old 12-09-07, 08:38 PM
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Besides some other good reasons already mentioned, the NFPA DOES base their recommendations on some scientific testing. Their goal is to make a recommendation which, if followed, will result in a very high percentage of smoke detectors always working. We know, and you have experienced, that a battery may very well last a couple of years. BUT, if we took say 1000 units and ran them for a long term test, we would probably see that at one year, a very high percentage of them are still working. As time goes on, that percentage starts to drop. This is how they come up with the one year recommentation.

The NFPA also recommends flat discarding all smoke alarms at the 10 year point. They found that older units have a 30% chance of failure within the first 10 years. That is a high percent, but combined with their recommendation for monthly testing to verify continued operation, the 10 year recommendation is reasonable. They do find that at 15 years, there is only a 50/50 chance a unit is still working, so don't dilly dally on the replacements!
 
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Old 12-09-07, 09:20 PM
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It's best to go directly to the CPSC web site. You'll find that their recommendations do not exactly align with how they have been summarized here.

For example, take this statement directly from their web site: "If a smoke alarm is battery operated, replace the batteries at least once a year to make sure the alarm will work when it is needed."

They also have information about new smoke detectors with a 10-year battery life.
 
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Old 12-10-07, 08:52 PM
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Just another suggestion that has served me well for a while...

Go ahead and replace the batteries once a year. Your house, valuables, family, etc. is worth it (having been through a devastating fire in the past). But use those still-good batteries in all those other battery-hogging devices in the house. RC Cars, remote controls, radios, etc. Especially if you have kids, there's no end to battery-sucking devices out there. And if they die unexpectedly, it's not a big deal...
 
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Old 12-24-07, 02:22 PM
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Battery life

I have 9 hardwired smoke detectors with 9V battery backup. I replace the batteries once a year just before Christmas. When I install new batteries the voltage is 10.17 VDC. The old batteries are 8.83 to 8.9 VDC. I would like to start tracking the voltage after a year of use to help determine when to replace them. I expect the voltage will decrease rapidly at some point.

Does anyone know at what volt level does the chirping begins?
 
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Old 12-24-07, 11:43 PM
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Alkaline batteries usually have a consistent rate of voltage decrease. Of course, temperature and load affect this; but generally they have a nearly linear drop in voltage.
 
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Old 12-25-07, 07:51 PM
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Originally Posted by Realist View Post
I have 9 hardwired smoke detectors with 9V battery backup. I replace the batteries once a year just before Christmas. When I install new batteries the voltage is 10.17 VDC. The old batteries are 8.83 to 8.9 VDC. I would like to start tracking the voltage after a year of use to help determine when to replace them. I expect the voltage will decrease rapidly at some point.

Does anyone know at what volt level does the chirping begins?
Sounds like a good project! I think you have reinforced the point that has been made:
(1) There is some variability in battery life span, due to manufacturing variances, storage time, and variability of the load device
(2) Murphy dictates that the person who is (a) unlucky enough or (b) clumsy enough, to end up with a fire in their premises, is the same person who does not graph battery voltage over time.

SO, the whole rationale to just change the darn battery every year is that it ups the odds that old Murphy's alarm will work just when he needs it most.

We can have a great philosophical discussion here about saving a few bucks by stretching battery life, and the conspiracy to sell more batteries, but that misses the point completely.
 
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