On Tue, 18 Aug 2015 23:15:51 +0100, alan_m wrote:

On 18/08/2015 22:36, Malcolm Race wrote:


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.


In my experience when the (alkaline) batteries start to fail in my smoke
alarms the chirping always starts at 2am/3am. Often ignored at night
the offending alarm cannot be identified in the morning after the sun
has come up and/or the central heating has kicked in.

The OP shouldn't overlook that there is also a battery in his digital
multimeter that may also be failing BUT readings tend to go high as a
DMM battery starts going flat.

That's *always* true on account the ADC reference voltage accuracy
becomes compromised by lack of voltage. The drop in the reference voltage
makes the test voltage appear higher than it really is. Normally, the DMM
will display 'Low Battery' well before the voltage reference becomes
compromised by an expiring battery.

I think most (if not all) DMMs choose to carry on displaying 'erroneous'
readings (along with the low battery warning) rather than simply refuse
to take any measurements under low battery conditions altogether as the
'Lesser of two evils' with the low battery warning offered by way of
mitigating this particular evil by warning the user that the accuracy may
well be compromised into a falsely higher reading, rather than leave the
user totally blind to the presence of (a potentially lethal) voltage in
the circuit under test.

ISTR that the 'standard' DMM test circuit impedances are 10 and 11
megohms on the DC voltage ranges (it may be higher on ranges above the
200v mark) whilst typical moving coil analogue meters can range from 5 to
50 kilohms per volt of the selected scale (20v FSD setting representing a
test meter impedance of 100 Kilohms to 1 Megohm) with 20 Kilohms per volt
being the more common (400 Kilohms meter impedance on a 20v FSD setting).

--
Johnny B Good