The problems I have had have been with brand new Duracell batteries; a temperature sensing device stops working when the voltage is down to 1.47 - similarly smoke alarms started beeping for 'low battery' when nominal 9V batteries were at 8.9V.

I am well aware that cheap batteries don't perform as well as quality ones, the question is why devices are not designed and built to be more tolerant and therefore be more economical in battery life?

If you read my posts carefully you will see that it is not a question about different specifications of batteries, it is about devices sold to the general public which do not call for special, heavy duty, high output batteries, but are supposed to work for a reasonable period on standard cells.

Why not design in more tolerance (or specify extra high duty batteries on the packaging)

Incidentally, I always read small print and instruction manuals.

Ohm's Law applies here, if the voltage is down, so will be the current, as it would be if the internal battery resistance increased or it developed a back EMF due to polarisation.

I posted the 'new' and 'used' voltages because open circuit voltage is easy to check with a multimeter as an indicator of battery condition.

madf, our smoke alarm batteries were also 9V (recently the local fire service installed alarms with batteries which last 10 years!) and had a habit of starting to beep at 5 am when the battery voltage had dropped as the temperature fell during the night:

A real PITA climbing step ladders to change them at that hour in dressing gown and only half awake.

Edited by galileo on 27/09/2011 at 17:44