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Smoke detector mystery

Discussion in 'General Electronics Discussion' started by BobK, Aug 7, 2014.

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  1. BobK


    Jan 5, 2010
    I have a smoke detector that is in a precarious place, on a sloping ceiling high above a flight of stairs. I cannot easily use a ladder to reach it because of the stairs, so the way I get to it to change the battery is by placing a sheet of plywood across the stair railing and a step ladder on top of that. My wife is terrified that I will take a 15 foot fall down the stairs.

    So she came up with a brilliant solution. Why not run a wire from the smoke detector down to a place we could reach without danger. I fitted the battery compartment with a 9V rectangular sized block of wood with terminals off a 9V and a speaker wire (probably 22 ga) leading out. Put in the battery and all was fine. For 1 day. It started beeping indicating the battery was bad. I replaced the batter and it lasted maybe a month. Next time I put a new fresh Duracell battery in and it indicated low battery right away. Putting the same battery directly in the smoke detector works fine.

    So what is the problem? Since these batteries will normally last 6 months, it cannot be pulling enough current that the 8 feet of wire makes a difference, can it? So I guess it comes down to how it tests the battery.

    I am now guessing that it pulls a relatively high current and checks the terminal voltage under that load periodically to ensure that it has enough juice to power the alert. And this would be done is a very short pulse, so, perhaps the inductance of the wires would limit the current for long enough for it to look bad.

    That is the first mystery.

    Just for kicks I put a 4700uF 25V capacitor across the battery terminals (yes the polarity was right). It did not help, and the capacitor got hot! I cannot imagine what would make the cap get hot, unless it was bad. So I put it across a 9V and then across my Lab supply set at 10V, and observed no heating.

    That is the second mystery.

    Any thoughts? Suggestions?

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Wow, that's a pretty deep mystery alright!

    I agree that the inductance and/or resistance of the wires would be the reason why the smoke detector reports battery low. I've had problems with smoke detectors being very fussy about the condition of the battery myself.

    I don't know why the capacitor gets hot though! It only happens when it's connected to the smoke detector, right? Is the smoke detector drawing very short bursts of very high current from the battery? What is the ripple current rating of the cap? Can you try one with a low ESR and a high ripple current rating?
  3. BobK


    Jan 5, 2010
    Yeah, ripple current would seem to be the only thing that would heat the cap that way.

  4. shrtrnd


    Jan 15, 2010
    I don't know how the detector circuit is wired, but I would assume that it operates (at low current) continuously.
    If you're using a thin gauge of wire for battery power, I could believe that the wire itself appears as a load in-line with the battery, to the smoke detector circuit.
    I suppose it's even possible that the smoke detector 'low battery' circuit could see enough of a voltage drop over a 15 foot distance of narrow gauge wire, to 'read' a low battery indication.
    We all know how hot a 9V battery gets when it's shorted. Maybe your cap appeared to be a short to your battery.
    No experience here, just speculation.
    I've got some 6 or 8 foot long (reel cord length) automatic reels I got years ago, supposed to be used to keep small tools up, and out of the way in a work area. They'll handle tools weighing up 4 lbs.
    Maybe you could look into something like that for your smoke detector. Mount the small reel assembly to the ceiling, and mount the smoke detector to the reel. If the detectior has a hook on it, you could hook the
    detector with a long hook on a pole, pull it down to change the battery, and then let the reel retract back up to it's position on the ceiling. Just a thought.
  5. Gryd3


    Jun 25, 2014
    This would be true, but is also dependant on the current draw of the smoke detector.
    Assuming 1000' of copper 22 guage stranded wire, 26mA would be required to drop 0.5V in the wire.
    Assuming 1200mAh, that only leaves us with a 48 hour operational lifetime for the smoke detector.
    Perhaps my math is wrong? But this does not seem to be the case. Something funny is going on. I'd like to stick a scope on it.
    This is true, but only momentarily when the cap is first connected and begins to charge.
    The op also stated that the capacitor did not get hot across the battery alone, or a 10V source... but only when connected to the smoke detector as well.
    Last edited: Aug 7, 2014
  6. BobK


    Jan 5, 2010
    We know the batteries last at least 6 months in normal use, so at 450mAH that would be an average current draw of 100uA! That is why I don't believe voltage drop in the wire could be responsible. I calculate 258 mOhms for 16 feet of 22 ga write, so at the average current, only a voltage drop of 25uV. At 1A it would drop 258mV which is probably just barely in the region that it might see as bad battery.

    I am going to put a shunt and my scope on it this weekend to see just how it behaves. This should be interesting.

    More info that I should have given before. The detector is connected to mains power. Whether it actually uses power from the main or not is unknown, because the main purpose of the mains connection is to communicate to the other detectors in the house so that they all sound the alarm. And I do have to change the batteries every 6 months, or, one by one they start chirping.

    KrisBlueNZ likes this.
  7. shrtrnd


    Jan 15, 2010
    As I said, I don't know how the detector circuit is wired. Don't you think you should be looking for competing circuit paths?
    Is this the only detector with the problem?
    Maybe the unit itself took a hit, or failed from natural causes.
    How did you run your 22 gauge wire to the unit? Any routing staples, or something that could be piercing the wire insulation?
    Any possibility of the mains ripple AC screwing with the battery circuit power circuit? (Inducing voltage/current changes)
    Just some ideas, which is what I thought was asked for.
  8. BobK


    Jan 5, 2010
    Not sure what you mean here.
    It is the only one that I have externalized the battery on, so yes. And it works fine with the same battery in the unit.
    No trauma to it.
    Yep, I used wire staples, but they have a good plastic insert so the wire is not scrunched or pierced.
    It is connected to mains either way, and it works without the battery external, but with the wiring for it still connected, so I don't think so.

    Thanks for your comments.

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