# How does the power supply of a smoke detector work ?

#### Roger at CCCC

Joined Jun 8, 2009
17
The smoke detectors in our house are connected to AC house power so that they continue to work even if the battery is removed. This is obvious because the green light on the battery detector stays on even after the battery is removed.

But where does the smoke detector get its power when the battery is still installed? It would make sense to me for the smoke alarm to always get its power from the AC house power, only using the battery when AC house power fails or is interrupted.

But the smoke alarm batteries typically have to be replaced once a year, even though AC house power has been available for almost all of this time.

So my question is: why does a smoke alarm apparently use battery power even when AC house power is almost always available?

Thanks for any response.

#### MikeML

Joined Oct 2, 2009
5,444
No, it just means #$%%$% lawyers got involved in writing the user manual...
The replace once a year is purely "cover your ass" verbige!

#### #12

Joined Nov 30, 2010
18,210
So my question is: why does a smoke alarm apparently use battery power even when AC house power is almost always available?
It doesn't.

#### mcgyvr

Joined Oct 15, 2009
5,394
I've noticed this too.. never looked into it though..

I have to replace the 9V batteries roughly every 3 years in my smoke detectors that are tied to the AC...
They start to "chirp" at me.. The batteries are dead/dying (tongue test)
I've never lost power more than 15 minutes total in the last 10 years..

#### #12

Joined Nov 30, 2010
18,210
They start to "chirp" at me.. The batteries are dead/dying (tongue test)
I just did the math. A 440 ma/hr battery can provide 16.7 microamps for 3 years. That is not even close to running an LED, so I have to believe the detector is running on house power and the battery just dies from internal leakage.

Then again, if the smoke detector only runs the LED for 1/1000th of a second per second, you could do this on the battery only.

Things that make you take stuff apart and measure it.

#### mcgyvr

Joined Oct 15, 2009
5,394
and the battery just dies from internal leakage.
That would be my guess too.. Didn't think sufficient leakage would occur in that time though.. but it may I guess
That and whatever or'ing scheme they are using is causing the battery and the ac derived dc supply to "share" the load under certain conditions..

Joined Jul 18, 2013
20,407
Obviously the battery is there in the event that the house power fails in the event of a fire, it therefore would have to be connected to the circuit somehow even if it were in a semi-isolated mode/state when the AC was present.
Max.

#### Roger at CCCC

Joined Jun 8, 2009
17
Thanks for all your replies. But internal leakage doesn't seem to me to explain why the smoke alarms using these batteries start to chirp after only a year if the batteries themselves are not used. The advertised shelf life of these batteries is usually five years, which suggests to me that they should last at least 3 or 4 years if they are rarely used. Since they DON'T typically last more than a year, that suggests that in fact smoke alarms run off of the battery, even if AC power is present and could be used. But what I wonder is whether anybody knows what the actual power circuit is that these smoke alarms use, or is there any way of finding out what it is ? Thanks again for all comments.

#### dl324

Joined Mar 30, 2015
10,488
But what I wonder is whether anybody knows what the actual power circuit is that these smoke alarms use, or is there any way of finding out what it is ?
It would be helpful if you stated the brand and model of your smoke detector.

This User Guide for a Kidde AC powered smoke detector states on page 13 that AC powered alarms (without battery backup) will not operate if the AC power has been cut off, such as by an electrical fire or an open fuse. This would lead one to believe that the battery is only used when AC power has failed.

Joined Nov 5, 2010
215
They may start chirping after a year because the mfgr has decided it's a good idea to replace batteries after 1 year because some people use really crappy batteries, and for legal reasons (as Mike said) they want to make sure the batteries are always fresh. It could be that the smoke alarm circuitry has a timer that times out a year after you put in the battery even if it's running on A/C and has the battery disconnected from the circuit (until power failure, etc.).

I have CO alarms (carbon monoxide) that time out in two or three years and display an End Of Life message due to the nature of the detectors. They must use an internal timer, so it's not that farfetched to think your smoke detectors could, also.

It would be better to alert at the end of 1 year rather than wait until the battery gets low enough to fail unpredictably. 1 year is long enough that it's not too onerous for an owner to buy a few 9V batteries on a yearly basis.

Joined Jul 18, 2013
20,407
It should also be very simple to test the small current that may be present from battery when AC is also connected?
Max.

#### dl324

Joined Mar 30, 2015
10,488
This topic prompted me to check one of my AC wired smoke detectors because I couldn't remember the last time I changed the battery; but I know it was longer than 2 years ago. I use NiMH batteries and the battery was down to 2V. My detector (Firex brand) has a green LED to indicated AC power present and an orange (probably red+green) that flashes periodically indicate backup battery is functioning.

I didn't think to see of the test function worked when the battery was dead and now I don't have a dead battery to test with...

#### marcf

Joined Dec 29, 2014
257
I could not resist this one. The biggest challenge was how to put the 9v battery in series with the smoke detector (First Alert) without destroying it.

Who knew that leggo blocks are very close to 9v battery dimensions. The terminals are 2-56 hardware.

I measured 1ma on battery power (AC power disconnected) and 0.1 micro amp (probably just noise) with AC power connected using a Fuke 97 DVM.

Battery was depleted however, suspect this is just normal 'shelf' depletion. I have not done so yet, but I will go over to the 'battery university'
website.

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Joined Nov 5, 2010
215
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I measured 1ma on battery power (AC power disconnected) and 0.1 micro amp (probably just noise) with AC power connected using a Fuke 97 DVM.

Battery was depleted however, suspect this is just normal 'shelf' depletion. I have not done so yet, but I will go over to the 'battery university'
website.
Can you restate this? I'm interested but I can't tell what you're saying exactly.

Like, what was the voltage on the 9V?

#### #12

Joined Nov 30, 2010
18,210
Can you restate this? I'm interested but I can't tell what you're saying exactly.

Like, what was the voltage on the 9V?
It was 9 volts.

There wouldn't be any sense in trying to measure the current from a dead battery, so he must have used a good one.

1ma would say the battery should last for 440 hours. 0.1 microamp says the battery should last 4.4 million hours. Therefore, you either have a warranty timer or the battery dies of its own accord in a year.

#### GopherT

Joined Nov 23, 2012
8,012
The smoke alarm tests the battery every few seconds to minutes depending on the brand. Mine looks to be a 0.1 second flash of low power LED every 10 seconds or so. 1% duty cycle and I would guess a 1 or 2 milliamperes.

We have to change the battery every 2 years (when it chirps). The chirp is loud and in the 22 years we have lived here, the first chirp is always between 1am and 5 am. Weird and annoying.

Joined Nov 5, 2010
215
The smoke alarm tests the battery every few seconds to minutes depending on the brand. Mine looks to be a 0.1 second flash of low power LED every 10 seconds or so. 1% duty cycle and I would guess a 1 or 2 milliamperes.

We have to change the battery every 2 years (when it chirps). The chirp is loud and in the 22 years we have lived here, the first chirp is always between 1am and 5 am. Weird and annoying.
Just because it flashes the LED doesn't mean it's checking the battery at that time. It may never check the battery and just operate by a timer.

#### GopherT

Joined Nov 23, 2012
8,012
Just because it flashes the LED doesn't mean it's checking the battery at that time. It may never check the battery and just operate by a timer.
Maybe. Maybe not.

What I do know, is that smoke detectors are the only thing in this house that uses 9V batteries. I have a partial pack of 9V batteries with the same date code as the battery in my smoke detector.

The batteries in the partial package all read 9 volts or more. The one in my smoke detector is sitting at 8.4 volts. It may not be the LED, but it's something.

#### #12

Joined Nov 30, 2010
18,210
smoke detectors are the only thing in this house that uses 9V batteries.
??? I have smoke alarms, 3 clocks, 2 voltmeters, and a phone answering machine that use 9V batteries.
Probably because most of my stuff is pretty old.

#### GopherT

Joined Nov 23, 2012
8,012
??? I have smoke alarms, 3 clocks, 2 voltmeters, and a phone answering machine that use 9V batteries.
Probably because most of my stuff is pretty old.
Your right, my Fluke meter uses a 9V. I turn it off when I'm done so I don't change it often.

Clocks use AA. Those AC clocks that have the back-up feature are empty. Phone uses a AAA pack of rechargeable batteries.