Eli5: Why do domestic fire alarms take a rectangular battery instead of AA/AAA?


Eli5: Why do domestic fire alarms take a rectangular battery instead of AA/AAA?

In: 12

Those rectangular batteries are 9 volts, AA batteries are 1.5 volts, so it would take 6 AA batteries to replace 1 rectangular battery.

I’m believe that a 9V rectangle is equivalent to 6 AA’s while being cheaper. So it’s compactness and cost for the power supply.

They need the higher voltage

A rectangular 9V battery is just 6 AAAA batteries in a box

If you want to get 9V from AA or AAA batteries then you still need 6 but now your battery compartment is longer and wider taking up wayyy more volume in the smoke detector making it a lot bigger

my smoke/CO alarm takes 2 AA’s. so this is incorrect. Its a first alert, so pretty common brand. in fact googling “smoke and co alarm 2 AA” yields a lot of domestic alarms that do. So there is no Eli5, you are just mistaken.

Nobody has used simple maths yet so here goes.

A standard disposable “Alkaline” 9v Square battery has 550 “Mah” of charge inside. Thats the size of the fuel tank if you like.

A Standard disposable “Alkaline 1.5v AA battery” has 2850 “Mah” of charge in it… so that’s effectively 5 times as big.

Because a smoke alarm uses so little energy each day (about 1 “Mah”) a 9v square battery is good for about 2 years of service in a battery powered smoke alarm. Unless it goes off on a regular basis because you burn your toast!

But 9v batteries are also used in a mains powered smoke alarm as a “backup” in case of a power cut and in these kinds of alarms they are probably using so little charge that a battery will probably last until the battery itself “fails” which can be up to 10 years after manufacture if it’s an “alkaline” battery. So in mains powered smoke alarms a 9v Battery will lasts the whole 10 year service life of the smoke alarm! Yes a standard smoke alarm will fail after 10 years or so because it’s detector “is radioactive” and it looses its radioactivity over time until it becomes unable to detect smoke!

Now onto the reasons why AA Cells are not used.

So “Alkaline” AA cells are so much bigger in (Mah) they could run a battery powered smoke alarm for about 10 years but they only have a service life of about 5-7 years from manufacture. This is because they are so “energy dense” that in a 9v smoke alarm actually they would actually begin to fail chemically long before they have used up all the charge which can be dangerous. Which is why the smaller 9v battery is safer as it will be empty before it fails chemically.

In a mains powered smoke alarm AA cells have a service life of 5-7 years and 9v cells have a service life of 7-10 years. So there is simply no point of putting a 6xAA batteries inside as they would probably need changing more often than a 9v battery does due to their internal chemistry is more fragile.

Fun fact in its original design there were 6 little circular battery rolls each about 1/3rd the size of an AA battery inside a square 9V battery case. So technically a 9v battery is actually 6 little batteries inside one bigger outside case. So you are actually putting “6x 1.5v Cells” inside your smoke alarm they are just all in one box and simpler to install.

Mah = “MilliAmp Hours” it just measures how much “fuel” a battery cell contains the standards for Alkaline cells looks like this. More modern cells by companies like Duracell can hold about 10% more charge these days than when the standards were originally drawn up due to improvements in battery design.

D Cells – 13,000 Mah

C Cells – 6,000 Mah

AA Cells – 2,500 Mah

AAA Cells – 1000 Mah

9 Volt Battery 500 Mah

Although it’s worth pointing out when smoke alarms were originally designed using a “radioactive detector” the 9v was necessary to power the detector. Many smoke alarms these days use a different type of detector that only use 3v so can be powered by 2xAA battery. Which design is best is a whole heap of science so let’s just say today different designs need different battery voltages and leave it there.