Building a 18650 Battery Pack: A Simple Guide
To build a battery pack using 18650 cells, you must first determine your power requirements. The voltage, capacity, and current requirements of your device dictate how many cells you need and how they should be connected.
1. Determine Voltage and Capacity
- Voltage: Connect cells in series to increase voltage. For example, to achieve a 24V system, connect 7 cells in series (3.7V per cell, nominal voltage).
- Capacity: Connect cells in parallel to increase capacity (amp hours). The number of cells in parallel determines how much current your pack can safely supply.
2. Configure Series and Parallel Connections
- Series Configuration: To meet your voltage requirement, connect cells in series. For a 24V system, you will need 7 cells (3.7V nominal each).
- Parallel Configuration: Determine how many cells in parallel are needed based on the current draw of your load. For example, if each cell can safely discharge 5A, calculate how many cells are required to supply the needed current for your load.
3. Choose the Right Cells
- Capacity: Choose cells with a high enough capacity to meet your desired runtime.
- Discharge Rate: Ensure cells can handle the required discharge rate (e.g., 5A per cell).
- Quality: Avoid cells with exaggerated capacities or poor quality. Test cells before use.
4. Spot Welding vs. Soldering
- Spot Welding: Use a spot welder to connect cells with pure nickel strips, as it minimizes heat damage.
- Soldering: If you must solder, use flux and a high-wattage soldering iron to avoid excessive heat. Soldering is less recommended for lithium-ion cells.
5. BMS (Battery Management System)
- A BMS is essential for protecting your cells from overcharging, overdischarge, and overcurrent conditions. It also helps balance the cells to ensure they all charge and discharge evenly.
- Connections: The BMS is connected to each series cell group and provides monitoring and protection for the entire pack.
6. Assembly and Testing
- Prepare: Test each cell’s voltage and ensure they are balanced before connecting them.
- Connect Cells: Use cell holders to organize the cells and connect them in series and parallel. Spot weld or solder the nickel strips.
- Install BMS: Connect the BMS to manage charging and discharging safely.
- Wrap and Insulate: Use heat shrink tubing to cover the battery pack, providing protection from moisture and physical damage.
- Test the Pack: After assembly, test the voltage and current output to ensure the pack works as intended.
7. Final Considerations
- Safety: Always ensure that your battery pack is properly protected with a BMS and that connections are secure.
- Maintenance: Regularly check the health of your battery pack and BMS to ensure safe operation.
By following these steps, you can create a reliable and safe 18650 battery pack tailored to your energy needs.
18650 Battery Pack Calculator and Planner
Notes:
Check The Cell Voltage
It is of critical importance to check the voltage of your cells before connecting them in parallel. Remember, if there is any voltage difference between the cells that you are connecting in parallel, energy will transfer from the higher voltage cell to the lower voltage cell as fast as it can. The difference in voltage is what determines the speed. So, the larger the voltage difference, the more the risk of a fire when you connect them.
While it’s not dangerous to connect out-of-balance cells in series, it certainly can be inconvenient. This is because it takes quite a bit of time before most consumer BMS systems are able to bring cells within balance, and until it does, you won’t be getting anywhere near the full performance of your battery pack.
While it’s still important to check your new cells, this is generally more of a problem when using used cells. As a general rule of thumb, you don’t want to connect two cells together in parallel if they are any more than .1 volts off.
Connecting:
A high-quality pure nickel strip with a thickness of 0.015 inches is usually the standard for building a battery pack with 18650 cells. At this thickness, each millimeter of width can carry about 1 amp of current.
