How to test a battery

Anyone know why the smoke detector batteries always give up in the middle of the night?

Beep. . . . . Beep. . . . Beep. . . .

 

They dont, its just that, that is when you hear them.

I thought rechargeables were not to be used in things that take ages to run down, like clocks or temperature sensors etc.
Probably wrong though.

 

Rechargeables have a faster self-discharge rate which means that even if you are not taking current they are still falling in voltage so they won't last as long before they fall below the minimum working voltage for the item they are powering. As you'll no doubt be aware the fully charged voltage of a rechargeable is lower than that of a non-rechargeable cell, 1.2V v 1.5V so with 2 NiMH in a temperature sensor the fully charged starting voltage will be 2.4V as opposed to 3V for a pair of Duracells and the NiMH cells will steadily self-discharge. And as has already been mentioned their voltage also falls as temperature falls.

 

Mine gets tested everytime I use my griddle pan and forget to close the kitchen door.