This is a simple charger for NiMH and NiCd rechargeable batteries, which
allows discharge the cells before recharging. NiCd batteries have a memory effect and so
need to be discharged to 0.9 Vbefore recharging, otherwise their capacity will significantly decrease.
In a battery powered device the cell won't usually discharge to 0.9 V as the device stops working much sooner.
Therefore, charger must ensure this function.
NiMH does not have to be discharged before each charge, but sometimes it is appropriate to do it
for maintenance or recovery of an older battery.
The charger charges two batteries in series. The principle of charging is simple: When you turn it on and the batteries are connected, press the button, it turns on the relay. This switches the charger to the discharge mode and each cell will discharge by current of about 0.4 A down to 0.9 V. When the voltage drops below 1.8 V (2 batteries are in series), the transistor and relay turns off. It switches the charger to charge mode. Charging current is about 0.1 A. The trimmer is used to fine-tune the threshold so that the relay turns off at 1.8 V voltage. Charging and discharging current can of course be varied by changing the resistance 100R and 5R6. When charging NiMH batteries without discharging, use the charger without pressing the button. The transistor can be BC547, BC548, BC237, BC238 ... The charger can of course be powered from a ready 12V DC power supply. If you build a power supply yourself, you have to keep the safety precautions of working with mains voltage.