Lead Acid Battery Charger

Introduction

This guide will walk you through building a Lead Acid Battery Charger that’s efficient, safe, and packed with features. Unlike traditional chargers, this project includes advanced protections against overload, overcharge, and reverse polarity, as well as LED indicators and real-time voltage and current display.

Lead acid batteries power many applications, from automotive to backup power. By building this charger, you can ensure safe charging for any 12V lead acid battery.

Features of the Charger

This charger design includes several key safety and usability features:

1. Overload Protection: Prevents excess current, which could damage the battery.
2. Overcharge Protection with Auto Cutoff: Automatically stops charging when the battery is full.
3. Reverse Polarity Protection: Safeguards against incorrect connections by cutting off power and sounding an alert.
4. LED Indicators: Shows charging, completion, and reverse polarity status.
5. Voltage and Current Display: Allows real-time monitoring during charging.

How It Works

The charger operates through three main stages to ensure a full charge without overcharging the battery.

1. Overload Protection

Using current-sensing resistors and the LM358 operational amplifier, the circuit continuously monitors the charging current. If the current exceeds safe levels, the charger reduces the flow to protect the battery from overload.

2. Overcharge Protection with Auto Cutoff

The charger measures the battery voltage and automatically cuts off charging once it reaches the optimal charge level (around 14.4V for a 12V battery). This prevents overcharging, which can damage battery health and lifespan.

3. Reverse Polarity Protection

Reverse polarity is detected using diodes. If the battery is connected incorrectly, a buzzer sounds, and a red LED indicates the problem, ensuring the charger does not power up and damage the components or battery

The schematic design for the Lead Acid Battery Charger in EasyEDA comprises the following four sections, each serving a specific function:

1. AC to DC Converter_SMPS

1. Purpose: Converts the 230V AC mains input to a stable DC voltage (14.4V) required by the charger.
2. Components: SMPS
3. Design Notes: A switching mode power supply (SMPS) approach can enhance efficiency compared to traditional linear regulators, providing stable output with reduced heat generation.

2. Constant Current Regulator

1. Purpose: Maintains a constant charging current, protecting the battery during the bulk charging stage.
2. Components:
3. LM358 Op-Amp: Monitors the voltage drop across a current-sensing resistor to regulate current flow.
4. TIP3055 Power Transistor: Handles high currents while maintaining output stability.
5. Resistor (0.5Ω, 5W): Senses current by producing a voltage proportional to the current flow.
6. Design Notes: The current sensing and regulation circuit ensures the charging current does not exceed the battery’s safe threshold, even if the battery is deeply discharged.

3. Automatic Cut-off Circuit

1. Purpose: Disconnect the battery from the charger once the voltage reaches 14.4V, preventing overcharging.
2. Components:
3. Relay: Automatically disconnects the battery when triggered.
4. Voltage Divider Circuit: Scales down the battery voltage for comparison.
5. LM358 Op-Amp: Compares the battery voltage with the reference voltage and activates the relay when the set threshold is reached.
6. Design Notes: The auto cut-off function ensures safe charging and enhances battery lifespan by stopping the process at the correct voltage level.

4. Reverse Polarity Protection Circuit

1. Purpose: Protects the charger and battery if the battery is connected with incorrect polarity.
2. Components:
3. Diodes (e.g., 1N4007): Prevents current flow in the wrong direction.
4. Buzzer: Alerts the user to incorrect connections.
5. LED Indicator: Lights up to visually indicate reverse polarity.
6. Design Notes: This circuit is crucial for user safety and prevents potential damage to sensitive components.

PCB Design Using the Toner Transfer Method

This method involves transferring the PCB layout onto a copper-clad board and etching it to create the circuit. Follow these steps:

1. Print the PCB Layout on Photo Paper

1. Export the PCB layout from EasyEDA in mirror image format.
2. Use a laser printer to print the layout onto glossy photo paper. Ensure the tracks and pads are clear and well-defined.
3. Cut the paper to fit the size of your copper-clad board.

2. Prepare the Copper-Clad Board

1. Clean the copper surface with fine sandpaper or steel wool to remove oxidation.
2. Wipe the copper surface using "Braso Metal Polish"
3. Wash the board with water and detergent, then dry it completely.

3. Transfer the Toner to the Copper Board

1. Place the printed side of the photo paper onto the copper surface.
2. Use an iron set to high temperature (no steam) to press the paper onto the board:
3. Press firmly and move the iron evenly for about 5–10 minutes.
4. Pay extra attention to the edges and small tracks to ensure complete transfer.

4. Remove the Paper

1. Allow the board to cool slightly.
2. Soak it in warm water for 5–10 minutes to soften the paper.
3. Gently rub off the paper, leaving the toner adhered to the copper.

5. Etch the PCB

1. Prepare an etching solution (Ferric Chloride is commonly used).
2. Submerge the board in the solution and agitate gently until the exposed copper is dissolved.
3. Rinse the board thoroughly with water to stop the etching process.

6. Remove the Toner

1. Use acetone or nail polish remover to clean the toner from the copper traces, revealing the final circuit.

7. Drill Component Holes

1. Use a small drill (e.g., 0.8mm or 1mm bit) to drill holes for through-hole components.
2. Verify hole alignment with the components before proceeding.

8. Inspect the PCB

1. Check for continuity in all traces using a multimeter.
2. Repair any breaks with a conductive pen or by soldering jumper wires.

Tips for Success

1. Ensure the photo paper is pressed evenly during ironing to avoid broken traces.
2. Work in a well-ventilated area during etching and toner removal.
3. Label the copper side with a marker for reference during assembly.

Enclosure Design Using Acrylic Boards

An enclosure protects the charger components and enhances its usability by providing a neat, durable, and safe housing. Here’s how the enclosure was designed:

1. Material Selection

1. Acrylic Sheets: Black acrylic sheets were chosen for their strength, lightweight properties, and aesthetic appeal.
2. Thickness: 2mm sheets ensure durability while being easy to cut and shape.

2. Designing the Layout

1. The layout was designed to:
2. Accommodate the PCB, SMPS, and other components.
3. Provide openings for the battery terminals, power switch, LEDs, and display.
4. Measurements were taken from the assembled PCB to ensure a snug fit.

3. Cutting the Acrylic Sheets

1. The design was transferred to the acrylic sheets using an acrylic cutter for precision.

4. Bending the Acrylic Sheets

1. Precisely mark the bending lines on the acrylic sheets.
2. Apply heat to the bending lines using hot air gun
3. Immediately bend the acrylic sheets precisely

5. Ventilation

1. Venting holes were added near heat-generating components like the SMPS and voltage regulator to ensure proper airflow and cooling.

6. Component Integration

1. LED Indicators: Openings were made for the charging and reverse polarity LEDs to be clearly visible.
2. Switches and Buttons: Slots were added for the power switch and charging mode selector.
3. Digital Display: The front panel included a cutout for the voltage and current display.
4. Battery Terminals: Protruding connectors were securely mounted for easy battery connection.

7. Finishing Touches

1. Edges were sanded smooth to avoid sharp corners.
2. Labels were applied near switches, LEDs, and terminals for clarity.
3. Protective rubber feet were added to the base for stability.

Tips for Acrylic Enclosure Design

1. Use masking tape on the acrylic surface during cutting to prevent scratches.
2. Ensure adequate space for wiring and component airflow.
3. Test-fit all components before final assembly to avoid misalignment.

Final Assembly

The final step involves integrating all components and ensuring the charger is fully functional and user-ready. Here’s the detailed process:

1. Prepare the Enclosure

1. Ensure the acrylic enclosure is fully assembled, with all cutouts and mounting points ready.
2. Clean the enclosure to remove dust or debris.

2. Mount the PCB

1. Secure the PCB inside the enclosure using standoffs to avoid direct contact with the enclosure walls.
2. Align the board so that LEDs, switches, and displays align with the corresponding cutouts.

3. Install Additional Components

1. Mount the transformer, relay, and other bulky components securely inside the enclosure.
2. Use cable ties or adhesive clips to organize wires neatly.

4. Connect the Wiring

1. Carefully connect all wires as per the schematic.
2. Connect the transformer output to the PCB input.
3. Link the LEDs, display, and switches to their respective terminals on the PCB.
4. Double-check connections to avoid polarity errors or loose wires.

5. Test the Circuit

1. Before sealing the enclosure, test the charger with a 12V lead acid battery:
2. Verify the LED indicators and display work correctly.
3. Check the current and voltage levels during charging.
4. Ensure the automatic cutoff and reverse polarity protection function properly.

6. Seal the Enclosure

1. After successful testing, attach the enclosure panels securely using screws or adhesive.
2. Ensure ventilation holes remain unobstructed.

7. Apply Finishing Touches

1. Label switches, terminals, and indicators for ease of use.
2. Add rubber feet for stability and aesthetics.

By following this guide, you’ll have a fully functional, safe, and reliable lead acid battery charger that extends battery life while protecting against common charging issues. Happy building, and remember to charge safely!