Can I replace a capacitor pack with a battery?

Hi.

We have mains smoke alarms in the shared area of our flats. One was beeping so I took out the capacitor pack (5 x Gold Cap 2.3V 10F) and plugged in a standard 9v battery. This shut the thing up but IS IT SAFE? I have no knowledge of capacitors or how they work but assume they're "one way" and not rechargeable - my concern is that the 9V will blow up or something on receiving current! Forgive my naivete.

I would like to replace the alarm but we have no idea where this thing is wired into, where the fuses are, etc (possibly in the flat of the guy who's never here)

But main question for now really just is - is this safe?

TIA.

Smoke alarms with capacitors for backup do not have replaceable parts, if they fail, the whole thing must be replaced.

By shoving a battery in there the alarm may or may not work, may or may not be dangerous, and now requires immediate replacement.

Can you put a 9volt battery in place of a 2.3 volt capacitor. Logically NO because the 2.3 volt rating suggest the capacitor is connected directly to a 2.3 volt ( or lower ) supply in the electronics. Connecting a 9 volt battery to that 2.3 supply rail is likely to damage the electronics.

That said the ciruit may survive a 9 volt supply for a short while depneding on the quality of the components and the design.

The capacitor pack is similar to a small rechargable battery ( part capacitive storage and part chemical storage ) and will provide power during a power cut. The advantage they have over a re-chargable ( chemical ) battery is that they can be connected directly to the power rails in the equipment without the need for a charging circuit.

dnjb said:

We have mains smoke alarms in the shared area of our flats.

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Which you are not qualified to work on.

I dread to think of the liability issues if people were to die because you'd been fiddling with the fire alarm system.

One was beeping so I took out the capacitor pack (5 x Gold Cap 2.3V 10F)

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5 caps this size?



Really?

This shut the thing up but IS IT SAFE?

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I have no knowledge of capacitors or how they work but assume they're "one way" and not rechargeable

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Actually, storing charge is exactly what they do.

https://en.wikipedia.org/wiki/Capacitor

my concern is that the 9V will blow up or something on receiving current

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Something could go wrong - it's not supposed to have a battery, and the one you've put in is not (I assume) supposed to have a voltage applied to it to recharge it. The "something" could be instant and permanent damage to the detector. I suggest you remove the battery forthwith.

I would like to replace the alarm but we have no idea where this thing is wired into, where the fuses are, etc (possibly in the flat of the guy who's never here)

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Don't fiddle with it yourself - speak to the freeholder/management company/resident's committee/as appropriate.

But main question for now really just is - is this safe?

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Not guaranteed to be.

bernardgreen said:

Can you put a 9volt battery in place of a 2.3 volt capacitor. Logically NO because the 2.3 volt rating suggest the capacitor is connected directly to a 2.3 volt ( or lower ) supply in the electronics.

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Not if they are in series.

dnjb said:

I took out the capacitor pack (5 x Gold Cap 2.3V 10F) and plugged in a standard 9v battery. This shut the thing up but IS IT SAFE?

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NO!

These supercapacitors are widely available; if you knew what you were doing and could be sure that your 9V battery had not caused damage, you could replace them with the correct parts. But do you even know that it was the capacitor that had failed in the first place?

Who originally installed these alarms? Is there an ongoing maintenance contract?

(I presume they are freehold flats, as you're in Aberdeen.)

Thanks for the advice folks - it's the reason I asked, because I wasn't sure it was safe. It stopped the beeping (it was beeping once with the capacitor pack connected, and twice with it disconnected) and the light was still flashing but yes, who knows what that means really - I may have dmaged something.

Anyway, I replaced it with a brand new one - didn't realise they were all a standard fitting, so it was just a case of unplugging it and plugging in the new one, no rewiring! Got one with a standard 9V battery so this drama doesn't happen again.

Thanks again for the advice and I do realise that messing with smoke alarms is not a great idea, but my "if I try this" head took over for a short time.

Oh and yes, there were 5 of those capacitors in the picture, in series, on a little bit of circuit board.

dnjb said:

Got one with a standard 9V battery so this drama doesn't happen again.

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Who is responsible for periodic replacement of the battery, and what record keeping procedures are in place?

Nobody is responsible for anything. There are no factors or anything like that - if something goes wrong then we all just chip in to get it fixed, except the guy who's never here. It will beep if/when the battery goes low and it can easily be replaced by anyone.

To be fair, I've stayed in flats where there were NO communal alarms so this is actually a step up!

A sensible person would check what their insurance policies say about it.

I accept my attempt to repair was ill advised. Hang my head on that one, fair dos - as I said, I simply had my "how can I fix it" head on for a while and it was a bad, bad idea.

As it turns out, a direct, brand new replacement alarm was a simple thing - no different from replacing an ordinary one, except it is plugged in. So, let's not go overboard, eh? If I had any doubts about it now I'd do something about it but it's really no different from sticking up a battery one bar attaching to plug.

Have you tested that it all works, including that smoke at one alarm causes the others to go off?

dnjb said:

As it turns out, a direct, brand new replacement alarm was a simple thing - no different from replacing an ordinary one, except it is plugged in. So, let's not go overboard, eh? If I had any doubts about it now I'd do something about it but it's really no different from sticking up a battery one bar attaching to plug.

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The point is that this is a shared fire detection system on which other people's lives depend.

What people do in their own homes is largely up to them, but you cannot have unqualified people fiddling with such a system. You should not have tried to repair it, you should not have replaced it. By installing one with a battery you have created a need for a schedule of battery replacement which needs to be a lot more formal than waiting until one of the residents gets fed up with the beeping. The system should be tested to ensure it still works as designed. It should probably be tested on a regular, formal, basis too.

Frankly I'd be very surprised if the law allows the situation which you seem to have.

Possibly in new flats the laws are much stricter about H&S issues but this is an old converted building. As I said, I've stayed at 3 flats prior to this where there were NO communal smoke alarms AT ALL - I suspect there is no legal onus or this surely wouldn't be the case.

I should not have tried to repair it, you're 100% right, and I have admitted that. Replacing it with a like for like (albeit with a battery for backup rather than capacitor pack) is debatable I would say. Given how simple it was, I'd say it was fine. Paying an electrician to, effectively, change a lightbulb is, IMHO, taking H&S to stupid levels of nannying. I actually only bothered in the first place because the girl upstairs (only 2 floors/4 flats to the building) asked me to have a look at it - I hadn't even noticed it beeping. The alarm outside my door, on the ground floor, is fine. Incidentally, it's mains powered but with a 9V battery for backup, just like the new one I bought.

I tested them both - they both work and they both sound if the other goes off. Honestly, that being the case, the fact that many flats have no communal alarms, the fact that it was like for like (bar backup type) - I'm not sure what you're after! I realise this is not something to mess lightly with, but it's also not rocket science! Almost no difference between that and people using a battery alarm only. You talk about "schedule of battery replacement which needs to be a lot more formal" - nonsense! When it beeps, somebody (probably me as I have a ladder) will put in a new one. Same as when a lightbulb goes - there is no formal arrangement and, again, there never was in any of the old flats I stayed in. I know newer properties often have factors, regular monthly fees for various things, but I've never stayed in one of those so they're obviously not the norm for older properties... there is just whatever arrangement the residents come up with themselves.

There must be someone/some "body" with legal responsibility for what happens in the common parts .Suppose a visitor falls because the stairs are defective/poorly maintained .Who does he sue, as he no doubt will? Will your neighbours sue you if they fall because you haven't changed a blown bulb?