On 18/08/2015 21:10, Graham. wrote:
On 18 Aug 2015 17:13:41 GMT, David wrote:

Just had the mains powered alarm chirping every now and then.
Puzzling in itself because although this should be a sign that the backup
battery is at the end of its life, why just chirp now and then?
Been doing it for the last couple of days, but only two or three chirps
every few hours.

Anyway, turned of the power to the alarms and took this one down.
Took the battery out and checked it with my old analogue meter and it
seemed to be at 9V.

Dug two possible spares out of the sod it drawer and tested them as well.
One showed around 9V and the other nearer to 10V.
Anyway, shoved the higher voltage one in and refitted and powered up the
alarm.
No chirping.
Will have to wait a day or so to confirm no intermittent chirping.

Decided to check the other batteries again with my cheapo ScrewFix digital
meter.
One showed a solid 8.85V. Further checks showed that it was out of date in
2010.
The one from the alarm initially showed 9.04V and 9.05V with the reading
flicking between the two.
Checking again, it showed 8.85V and the voltage started to slowly decline
as I kept the meter on it.

Puzzled, I tried by analogue meter on it again.
Solid around 9V and no decline.

Tried the digital again and it showed 9.04/5V.

Tried it again and got the declining reading.

So it seems the battery was at the end of life and declining slowly, but a
spot reading indicated it was fine. I thought when they went, they went
down and stayed down.

Checking what the nominal voltage should be (I assume that this is the
same as that read by a meter) WikiPedia suggests 9V for Alkaline and Zinc
Carbon with 9.6V for Lithium.
https://en.wikipedia.org/wiki/Nine-volt_battery
So the solid 8.85V is probably toast for anything that is very voltage
sensitive although the same article says "nearly dead" is around 5V and
most devices are designed to cope with this voltage range.


As you can gather, I am puzzled by the erratic behaviour of this
particular battery.
Is this normal for 9V batteries?

Cheers


Dave R

Yes.

What can happen is the ambient temperature drops, causing the terminal
voltage to droop below the "chirp" threshold.
The initial chirp draws current causing the voltage to reduce more and
the chirps continue in a vicious circle. A rise in temperature may be
enough to break the loop, but not always.
I've seen this particularly when lithium 9V batteries are used in
smoke alarms.

Not only Litium. 20 approx years ago my elderly mother was disturbed at
night by chirping noises which she thought came from her phone. She
reporrted a fault to BT and one of the first questios asked was 'Do you
have a smoke alarm?' She did, close to the phone. The problem was an
alkaline battery dropping in voltage as the temprtature dropped at night.

Malcolm