The circuit shown in the schematic is built to charge 12V batteries but you can easily adjust it to automatically charge other voltage batteries. For charging other voltage batteries first you should know the max charging point of the battery you want to charge. For example a 12V lead acid battery shows 14.4 on the multimeter during charging when it becomes full charge and a 6V battery shows 7.2.
For adjusting the circuit for 12V lead acid batteries connect an adjustable power supply in the place of battery and set it to 14.4V. Now adjust the 10K variable resistor and stop on the point where the green LED becomes activated.
For adjusting the to charge 6V lead acid battery first decrease the value of 9.1V zener diode to 3V zener diode. Now connect a adjustable power supply and set it to 7.2V and connect it in the place of battery. Now adjust the 10K variable resistor and stop on the point where the green LED becomes activated.
The same procedure can be applied to set the circuit to charge other voltage batteries. The zener diode value should be around half of the battery voltage.
The circuit can also be used to charge high AH batteries. To charge high AH batteries use a 5A to 10A transformer and a 5A to 10A diode bridge. The supply voltage coming from the diode and filter capacitor to the battery should be not more than 2V to 3V higher than the battery voltage. For example for a 12V battery the supply voltage should be 14.5V to 15V. for 6V battery the supply voltage should be not more than 8V to 9V.
Here is a very simple automatic 12V and 6V battery charger circuit with auto cut off relay. The term auto cut off means that the circuit will automatically cut the battery with the charger circuit when the battery will become full charged. The circuit is using only two transistors and few other discrete components.
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