On Tue, 18 Aug 2015 20:01:37 +0100, Toby wrote:

On 18/08/2015 18:13, 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 Further checks showed that it was out of date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



A digital meter presents almost no load on the battery, where as the
analogue one presents a load - if you test the battery with the digital
and analogue meter at the same time, it will almost certainly show thwe
voltage dropping on both.

So at no load, the voltage seems OK-ish, but when presented with a load,
it drops.

Except that the analogue meter showed no slowly dropping voltage - it was
the digital one which showed the strange behaviour.

Thanks to all for the explanations.


Cheers

Dave R


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