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
AC powered Smoke alarms and their batteries
Mar 05, 2005
5 Replies
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.)
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
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. EricJoin the Discussion
Have something to add? Share your thoughts — no account required.
Didn't find your answer?
Ask the community — no account required