Note- You can use any Li-ion charger which is used to charge phone's 3.7v Battery as they both are charged with a 4.2V DC. but if you still want to make one of your own. just go through this instructable.

Here's a step-by-step guide to making a 4V lead acid battery charger using a 5V regulated supply:

1. Gather the materials needed for the project:

- 5V voltage regulator (such as L7805)

- Header pins or connectors for connecting the battery to the circuit

- 1N4001 rectifier diode

- ECB (Experimental Circuit Board), breadboard, or any other suitable prototyping board

2. Begin by connecting the components on the ECB or breadboard:

- Insert the 5V voltage regulator (L7805) onto the board, ensuring it is oriented correctly.

- Connect the input pin (Vin) of the voltage regulator to the positive terminal of your 5V regulated power supply.

- Connect the ground pin (GND) of the voltage regulator to the negative terminal of the power supply.

- Connect the output pin (Vout) of the voltage regulator to one end of the rectifier diode (1N4001).

- Connect the other end of the rectifier diode to the positive terminal of the lead acid battery.

- Finally, connect the negative terminal of the battery to the ground (GND) of the power supply.

3. Once all the connections are made, double-check them for accuracy and ensure there are no loose or shorted wires.

4. Plug in the 5V regulated power supply to a suitable power source.

5. The voltage regulator will regulate the input voltage to a steady 5V output, which will charge the lead acid battery connected to it. The rectifier diode prevents the battery from discharging back into the power supply.

Note: It's important to monitor the charging process and ensure the battery does not overcharge. Lead acid batteries typically require a specific charging profile to avoid damage or reduced lifespan. If you're unsure about the charging requirements for your specific battery, it's recommended to consult the manufacturer's guidelines or use a dedicated lead acid battery charger.

Remember to exercise caution when working with electricity and follow safety precautions to prevent any accidents.

Step 1: The Circuit

To ensure proper functioning of the battery charger, it is essential to have a DC power source with a voltage greater than 7 volts. You have a couple of options to obtain such a power source. One option is to connect a laptop charger with a DC jack, while another option is to utilize a set-top box adapter. Once you have secured the appropriate power source, you can proceed with connecting the battery to the output of the voltage regulator.

For a more detailed visual representation of the circuit, please refer to the provided pictures.