Power consumption of a smoke alarm

[skylark42]

I need to install new batteries on my bog-standard, inexpensive, ceiling-mounted smoke alarms, which are chirping. Both take 9V batteries and I want to replace them with rechargeable ones. I have established that:

1. I can get reasonably-priced NiMH batteries.
2. Rechargeable "9V" batteries are in fact 8.4V, but this is sufficient for a smoke alarm.
3. Most inexpensive rechargeables are 160 mAh.

Question is, how long will this last? I don't want to be getting up the 10ft ladder every 2 months. I have looked at the smoke alarm info available online but power consumption doesn't seem to be something they publish.

Thanks!

 

[motorbiking]

I used these on the sensors on my boat and motorhome
https://www.7dayshop.com/9v-pp3-rechargeable-batteries
They claim to be 250mAh
They have lasted over a year on a charge so far

 

[sparkymarka]

Don't bother fit new smokes with 10 year lithium batteries ....only £ 12.99 screwfix

 

[Keithmac]

We got our smoke alarms free from the Fire Brigade..

 

[Doggit]

And if they're still giving out Fire Angel, be careful, as they don't work properly. They take ages to react, and sometimes don't; and they they used to be advertised as a 10 year backup battery, and are now down to 7.

Now, do you want free, cheap, or thank God I fitted the good ones.

 

[sparkymarka]

That bad batch has been and gone ....

 

[skylark42]

Thanks all for your input but what I really want to know is how much power a smoke alarm uses, hence how long a 160mAh battery might last (i.e. if it is in 'watch' mode only and doesn't have to ring any alarms).

 

[rsgaz]

how much power a smoke alarm uses

This one uses 150microamps on standby, from a 12v power source....
https://www.fastlec.co.uk/ei100r-aico-ei100r-12v-dc-powered-smoke-alarm.html

So, 0.0018 watts. I have no idea whether that's a typical value/very high value/very low value/etc.

 

[rsgaz]

I think that Aico one I linked to above must be a particularly high current, because i have found a battery operated CO detector that consumes only 20microamps on standby...

https://testproducts.com/costar-wall-ceiling-carbon-monoxide-alarm-battery-operated-p-1664.html

So, a tenth of the power of the Aico detector, 0.00018 watts.

 

[skylark42]

Right, RSGAZ, so here goes on the time the battery will last.

A 160 mAh battery can supply 160 mA for 1 hour, or 160 microA for 1000 hours.
So - using your CO monitor consumption - if it's supplying 20 microA, it should last 8000 hours, or 333 days. That's a year in old money. I'd be happy with that.

(If the 150 microA one is 12V, it's probably a mains one plugged into an adaptor, i.e. not the stick-it-on-the-ceiling type that I have.)

Thanks very much for your research and figs!

 

[rsgaz]

That's a year in old money. I'd be happy with that.

It will start chirping way before that though, you're starting from a lower voltage than 9 to begin with.

6 months would be a reasonable guesstimate.

 

[skylark42]

Well-reminded. Thanks. Any idea what the effect of voltage lowering will be? I'm struggling to think how fast 'charge evaporation' will affect the voltage, and hence how soon it will start chirping.