The automatic NiCd battery charger makes use of the properties of a Type 555 timer. The internal widow comparator of the IC is set to 4.7 V by zener diode D1. If the potential at pin 6 exceeds that level, the output voltage at pin 3 becomes low. If at the potential at pin 2 drops below half the reference voltage, that is 4.7/2 = 2.35 V the output voltage becomes high.
How NiCd battery Charger works?
When the voltage of the battery, to be charged is low, and the output potential of IC1 is thus high, the battery will be charged via R6 and D2 until the ‘fully charged’ voltage, set with P2 is reached. The output of the IC then changes state and charging ceases. However, the IC continues monitoring the battery voltage with the aid of T1. When the output of the battery, owing to self-discharge, has dropped below the level set with P2, the IC causes charging to recommence. Dual LED D3 lights red when charging is in progress and green during the quiescent periods.
NiCd battery Charger Circuit Diagram:
Salient feature of NiCd battery Charger:
In this way, the battery can be left in the charger for any period you like: it will always be fully charged ready for use when required. It will never be overcharged, nor will it be discharged more than a little. With values as shown in the diagram, the charger is suitable for 6 V and 9 V batteries. Nine-volt types with 6 or 7 cells are charged at about 20 mA. P1 should be set to arrange for the charger to switch off after 14 hours. The lower window level is set to about 1 V below that value with P2.
Six-volt types with 4 or 5 cells are charged at 55 mA. Again, P1 should be set so that the charger switches off after about 14 hours. The lower window level is set to about 0.8 V below that value.
Precautions while using NiCd Battery:
It should be noted that NiCd battery manufacturers do not recommend ‘fully charged’ voltage measurements, but rather time or temperature measurements. Nevertheless, the prototype of the present circuit has been used for charging small NiCd batteries to good effect for some time.