In modern electronics, reliable power sources are crucial for various applications, ranging from portable devices to DIY projects. Constructing a custom battery pack allows for flexibility in voltage and capacity, making it suitable for specific needs. This report outlines the steps to create a 12V, 4000mAh battery pack using lithium iron phosphate (LiFePO4) cells, which offer high energy density, safety, and longevity compared to other lithium-ion batteries. This battery pack will provide a stable power supply for electronics requiring 12V.
Materials Required
- 26650 3.2V 4000mAh Li-ion Rechargeable Battery 3C Cell x 4
- 4S BMS for LiFePO4 cells (with balancing and protection features)
- Epoxy Sheets for making a shell or box for the battery pack.
- Nickel Strip for connecting cells
- Spot Welding Machine
- 2-Pin SPST On-Off-On Rocker Switch
- Battery Capacity Display
- Female DC Jack
- Wires
- Battery Support Brackets
- Multimeter
- Insulation tape for the box
- Insulation tape for the circuit and battery pack
- Soldering Iron
- Heat Shrink Sleeves
- Heating Gun
Step By Step Guide to Build the Battery Pack
Step 1: Assemble the Battery Pack
- Arrange the Cells:
- Align the 4 cells in series to achieve the desired voltage (4 x 3.2V = 12.8V nominal, approximately 13V when fully charged)
- Connect the Cells:
- Use a spot welder to attach nickel strips between the positive terminal of one cell and the negative terminal of the next cell in the series with the help of nickel strips through the spot welder.
- Ensure connections are strong and secure.
- Balance Wires (for BMS):
- Connect balance wires from each cell to the corresponding BMS balance terminals (B1, B2, B3, B4).
Step 2: Connect the Battery Management System (BMS)
- Attach BMS:
- Connect the BMS to the battery pack:
- B+ to the positive terminal of the first cell.
- B- to the negative terminal of the last cell.
- Connect B1 terminal of BMS to the intersection of B1 and B2.
- Connect B2 terminal of BMS to the intersection of B2 and B3.
- Connect B3 terminal of BMS to the intersection of B3 and B4.
- P+ and P- serve as the output terminals for switch, capacity display and DC jack for charging.
Step 3: Add the Capacity Display and Switch
- Connect the Capacity Display:
- Connect the display positive to P+ and negative to one end of the switch.
- Wire the Rocker Switch:
- Connect the switch to control power output:
- Connect one terminal of the switch to the negative output of the battery (P-).
- Connect one terminal to the negative wires of display and jack
- Connect the DC jack:
- Connect the positive wire of the jack to the positive output of the BMS (P+).
- Connect the negative wire to the other end of the rocket switch.
Step 4: Final Assembly
- Test the Battery Pack:
- Use a multimeter to verify the output voltage is approximately 13V.
- Check that all connections are secure and functioning properly.
- Secure the Enclosure:
- Place the assembled battery pack into the holder or enclosure, ensuring all components are securely fastened and insulated.
- Insulate and Protect:
- Use insulation tape and heat shrink tubing for additional safety.
- Cover the battery pack and components with heat shrink sleeves and use a heating gun to shrink the sleeves, ensuring a snug fit.
Step 5: Testing and Validation
- Initial Charging:
- Charge the battery pack using a compatible charger for LiFePO4 cells.
- Ensure the charger provides the correct voltage and current.
- Load Testing:
- Connect a load to test the battery pack’s performance under real conditions.
- Monitor the voltage and capacity display to ensure the pack is functioning as expected.
Technical Considerations
- BMS Role: The BMS ensures cell balancing, overcharge, and over-discharge protection, enhancing the battery pack's lifespan and safety.
- Safety: Use appropriate safety measures when working with lithium batteries, including wearing safety equipment and ensuring proper ventilation.
Conclusion
Building a custom 12V 4000mAh battery pack offers a tailored power solution for various applications. By following the outlined steps and utilizing LiFePO4 cells, this project delivers a reliable and efficient energy source. The integration of a BMS and other components ensures the battery pack's safety, performance, and longevity. Proper assembly and testing validate the battery pack's capability to meet the required specifications.