AC powered Smoke alarms and their batteries

I have AC powered units in my house and each has a 9v battery in it. Why is it the battery doesn't last for several years? Isnt it only supposed to be for backup if the AC goes out? What drains it other than that? And also, why dont they make a similar unit that would use a rechargeable 9v? (and keep it charged up). Eric
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They get you both ways. No smoke alarm lasts for a decade (because the Americium deteriorates.) No 9V battery continues to maintain a 9V p.d. for "several years" because its voltage declines. (Cost of building a battery recharger into the unit would be prohibitive.)
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Its self-discharge may be limiting lifetime... 1%/year for alkaline, 10%/year for carbon zinc, 1%/day for NiCd or NiMH...
You might keep a 400 mAh carbon-zinc battery charged by supplying 0.1x400mAh = 40 mAh/year, ie 4.6 uA, on a continuous basis, using a charge pump with 60x340C = 4.6x10^-6, ie C = 220 puffs.
Nick
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Depends on quality of battery; makes like Duracell alkaline will in back up mode work 3.5 to 4 years. Cheap zinc carbon will be only 12 mths.
Are you sure it is receiving mains power? Decent makes show a green led when mains is connected.
Regards later post, smoke alarms should be changed at 10 years or before.
Loose Rechargeables should not be used [except type supplied with built in rechargable cells]as their discharge characteristics will not run the low battery signal as required under the Br Standard.

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I have several smoke alarms that are at least as old as 10 years; they do sound off; occasionally. The one between kitchen and bedrooms if something overheats on the stove or in oven. (Also once on a very humid day) The one in the storeroom by door to garage if an engine is run in attached garage. The one above basement workshop door if one inadvertently or carelessly opens wood stove door to add wood and smoke puffs out. So I've assumed that they are all working OK? But should I be concerned that they are running down due to decay of the radioactive isotope? In other words should I replace? BTW one of the working ones and an unused spare were bought very cheap for around $3 at a well known store. I'm wondering now it they were perhaps "Beyond date". i.e. Maybe beyond or approaching that ten year service life? Comments appreciated. Terry
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Gel wrote:

Maybe thats why its been a year and a half and I'm still waiting for them to beep "low batt" but they havnt, i replaced the old "el-cheapo" batteries at that time with duracell's.

Yup, no doubt they are powered and working normally - i just wondered why the batteries drained - perhaps there is some kind of internal "bat check" every so often that applies a brief load and verifies voltage is above xx. If that were the case then over time it would drain the batteries.

Why? The half life of Americium-241 used in smoke detectors is 458 years - in 10 years it will have decayed by only 2.1%, hardly enough to have much effect on operation. Eric

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