you have to change the value of the 3.3V zener diode to 2.4V or 2.5V. To set the circuit to charge your desired battery select the power supply atleast 1.5V to 3V above from the battery under charge. For example if you want charge a 6V battery, the power supply should be around 7.5V DC. For charging 9V battery use 11.5V to 12V power supply and for charging 12V batteries use 14.5V to 15V power supply. The current of the power supply should be selected according to the 1/10 of the battery AH. For example if you want to charge 6V 10AH battery then use 1A current power supply and if you want charge 6V 100AH battery use 10A current power supply.

The 100K variable resistor is used to set the trip point of the circuit where it will stop charging and disconnect the battery from the power supply. The trip point should be set according to the battery type for example a 12V battery will show 14.4V on the digital millimeter with the power supply connected when it is fully charged and a 6V battery will show 7.2V with the power supply connected when it is fully charged.

Now for setting the circuit to cutoff or disconnect the battery on the required voltage you have to adjust the circuit with a variable power supply. For example if you want to charge a 12V battery and want to disconnect the battery on 14.4V then set 14.4V on the variable power supply and remove any power supply or battery attach with the circuit. Now connect that 14.4V variable power supply in the place of battery and slightly adjust the 100K variable resistor until the LED turns ON. After these adjustments are done remove the variable power supply and connect the actual power supply and battery of the circuit. Now when your 12V will be fully charged the circuit will disconnect it from the supply and the LED will become switch on to show that the battery is fully charged.

The same procedure can be applied to set the circuit to charge other voltage rechargeable batteries but the variable power supply voltage should be changed according to the full charge voltage of the battery. Like you have to select 7.2V in the variable power supply to set the circuit for 6V rechargeable batteries and then adjust the 100K variable resistor at the point where the LED turns ON.
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Here is a simple and efficient project of a 555 universal automatic battery charger circuit. The circuit can be adjusted to automatically charge any type of rechargeable battery From 6V to 24V and supply 10A max current. It can also be used to charge batteries below 6V, for this purpose only
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555 Universal Automatic Battery Charger Circuit Diagram