Schematic Reference

Hyper-Simple Battery Capacity Tester

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 1.2V/5.6R = 214mA. 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.