If you have a motley
collection of 12V batteries in varying states of health, this simple
circuit will allow you to easily check their capacity. It’s basically a
high-current discharge load which is controlled by the NiCd Discharger.
This involved increasing the existing 10µF capacitor across LED1 to
100µF, to enable it to supply the brief current pulses required by the
clock mechanism. The discharger’s “clock connection” now controls a
BC457/BD139 Darlington transistor pair (Q1 & Q2) via a 1kO resistor.
These in turn activate a car headlamp relay to switch in a preselected
lamp load (one of three).
With 12V selected, the prototype unit stops the discharge at 11.4V
which corresponds to a cell voltage of 1.9V (this is a pretty good
indication of a discharged 12V battery). The loads consist of three
automotive lamps, selected to provide discharge rates to suit the
battery being tested. These lamps should be fitted to sockets, so that
they can be easily swapped for other lamps with different wattages, if
required. That way, the discharge current can be varied simply by
changing the lamp wattage. By the way, this circuit will also work with
6V batteries, provided the relay holds in. This gives an “end-point”
voltage of about 5.7-5.8V.
Author: Reg Carter – Copyright: Silicon Chip Electronics