Battery for smoke alarm: Zn-Cl or Mn-alkaline?

[Clive Page]

While lying awake last night I heard short beeps at long intervals, almost certainly a smoke alarm trying to tell me its battery was low, but I couldn't be bothered to get up to find which one (of two) it was. In the morning, by some variant of Murphy's Law, the beeping had stopped but both worked perfectly well when I pressed their test buttons. Both have PP3 batteries currently showing about 8.5 volts, so I've still no idea which one it was, but I'll replace both to be on the safe side.

Although manganese alkaline batteries are better value for most things, I have a vague memory that zinc chloride ones actually last longer for very low consumption tasks, such as door bells and smoke alarms. Is this right?

[Andrew Gabriel]

For smoke alarms on PP3's, I use alkaline, and replace annually. They last longer than a year, but not 2 years, but it's easy to nominate a date, such as your birthday, or Christmas, when you routinely replace them.

I did buy some of the long life lithum PP3 cells, and they worked for a year or two, but then the detectors started bleeping. However, the batteries are 11V, so I presume this is due to too high voltage, although I'm not sure why they worked for some time before that.

Most of mine run off the burglar alarm supply, so there are no batteries to change in them, but I look after various battery ones around the family.

[Brian Gaff]

To be honest, I've not noticed much difference except that some seem to be harder to push the connectors onto!

Brian

[Brian Gaff]

Over voltage is not a problem. the problem is internal resistance, ie they may measure high on small loads, but when they do a test they momentarily take more current, ie with the bleeper and hence their voltage starts to drop due to the internal resistance. Brian

[Scott]

I'm sure mine last far longer than two years. If you buy ones that don't have replaceable batteries I thought they lasted five years. Why would quality batteries be any different?

How can they be too high a voltage? Are batteries not 12 Volts nominal?

I assume your burglar alarm system has a battery to cover power failures.

[Roger Hayter]

I remember that being the received wisdom, but the specified shelf life of alkaline batteries seems so long ahead nowadays that I wonder if it is (still) true?

[alan_m]

It's temperature. During the night the battery gets colder and the terminal voltage drops.

[alan_m]

Fresh Duracell Industrial (formerly Procell) alkaline batteries are dated 2024.

[Bob Eager]

The PP3 is a 9 volt battery.

[Scott]

Good point. Apologies for introducing fake news to this discussion :-)

[Mr Pounder Esquire]

That would explain why the clock in my garage lost time when it was very cold and was fine when it got warmer. I am enlightened.

[dennis]

Because they are not alkaline batteries.

Mine are supposed to last 10 years, I will let you know in 6 years if I remember.

[tabbypurr]

t certainly a smoke alarm trying to tell me its battery was low, but I coul dn't be bothered to get up to find which one (of two) it was. In the morni ng, by some variant of Murphy's Law, the beeping had stopped but both worke d perfectly well when I pressed their test buttons. Both have PP3 batterie s currently showing about 8.5 volts, so I've still no idea which one it was , but I'll replace both to be on the safe side.

have a vague memory that zinc chloride ones actually last longer for very low consumption tasks, such as door bells and smoke alarms. Is this right?

I don't think so. ZnC/Cl leak when flat too.

NT

[Andrew Gabriel]

10 years I think, which is because they have built-in non- recharable lithium cells.

They're 9V labeled, but they have 3 non-rechargable lithium cells in them which are nearly 4V each, so they're well over 9V. It does amuse me that it says "do not incinerate" on them ;-)

Yes, 12V. The smoke alarms are standard 9V PP3 battery alarms, but instead of the battery, there's a PP3 sized module which drops the 12V to 9V and measures the current. When the current increases due to the sounder, the module signals back to the alarm panel that the smoke sensor has triggered.

[Clive Page]

I thought it might be. It's surprising that a degree or two would make that much difference though.

[ARW]

It only happens when the batteries are on their last legs.

But you made the right decision by deciding to swap the batteries in both smokes and not f*ck about deciding if one battery is better than the other one.