Eli5: Why do domestic fire alarms take a rectangular battery instead of AA/AAA?
In: 12
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.
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.
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.
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.
Like any mass market consumer product, companies seek to reduce the costs of their products as much as possible. One way to do this is to reduce the number of components required and so smoke detector chips such as the NXP MC145017 were created that integrated many of the components of a smoke detector in a single device. These chips have a recommended input voltage of 9V so the easiest (and cheapest because a 6xAA batter holder costs more money) to get the recommended 9V is one 9V battery.
Like any mass market consumer product, companies seek to reduce the costs of their products as much as possible. One way to do this is to reduce the number of components required and so smoke detector chips such as the NXP MC145017 were created that integrated many of the components of a smoke detector in a single device. These chips have a recommended input voltage of 9V so the easiest (and cheapest because a 6xAA batter holder costs more money) to get the recommended 9V is one 9V battery.
Like any mass market consumer product, companies seek to reduce the costs of their products as much as possible. One way to do this is to reduce the number of components required and so smoke detector chips such as the NXP MC145017 were created that integrated many of the components of a smoke detector in a single device. These chips have a recommended input voltage of 9V so the easiest (and cheapest because a 6xAA batter holder costs more money) to get the recommended 9V is one 9V battery.
There absolutely are smoke detectors that take AA batteries. Mine take 3 AA batteries each. I prefer it that way because I change the batteries at the beginning of the year regardless if they need to be changed or not. Then I just put those batteries into rotation for tv remotes or flashlights or whatever.
There absolutely are smoke detectors that take AA batteries. Mine take 3 AA batteries each. I prefer it that way because I change the batteries at the beginning of the year regardless if they need to be changed or not. Then I just put those batteries into rotation for tv remotes or flashlights or whatever.
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