The circuit described
here is eminently suitable to indicate the capacity of a battery. We use
a cheap electric clock for this. By connecting a resistor across the
battery terminals, the battery is discharged somewhat faster than with
the clock alone. If we pick a resistor with a value of 5.6R, the
discharge current amounts to 1.2 V / 5.6 R = 214 mA. If we multiply this
with the number of hours that the clock ran after the battery was
connected up then we know (approximately) the capacity of the battery.
When discharging a NiCd battery we need to make sure we remove the
battery the moment the clock stops running. NiCd batteries do not
tolerate too deep a discharge very well. We therefore recommend keeping
an eye on the voltage in one way or another, for example by connecting a
multimeter in parallel with the resistor.
Author: J. Van der Sterre
Copyright: Elektor Electronics