How to Assemble Lithium Battery Cells into Battery Pack

The process of assembling lithium battery cells into a complete battery pack is called battery PACK. It involves integrating key parts such as the cells themselves, a protection board, nickel strips, and labeling materials. Through a series of processing and assembly steps, these components are combined into a fully functional battery pack.

Let’s go step by step and see how a lithium battery pack is designed and assembled.

What is a Lithium Battery Pack?

A lithium battery pack is essentially a circuit system made up of multiple lithium cells that work together to store and deliver energy.

Each pack can contain several cells, and each cell comes with its own voltage and capacity. By connecting cells in series or parallel, the pack can be configured to deliver either higher voltage or greater capacity. A charging system is typically built into the pack to supply energy to devices.

The size of a pack depends on the number and type of cells it contains. Common formats include 18650, 21700, 26650, 32700, and prismatic cells. The number of cells used directly influences the pack’s final size.

Step 1: Cell Sorting and Grouping

The first step is to sort and match the cells. Technicians group cells with similar characteristics to ensure consistency. Key parameters include:

• Internal resistance
• Voltage
• Rated capacity
• Charge/discharge efficiency
• Self-discharge rate

By grouping similar cells together, internal balance is improved, efficiency is higher, and the battery pack’s service life is extended.

Step 2: Cell Assembly and Fixture Alignment

The sorted cells are placed into structured fixtures and then spot welded together. Following the Standard Operating Procedure (SOP) is critical at this stage, especially ensuring correct polarity alignment (positive and negative terminals).

Proper alignment avoids short circuits and ensures electrical safety.

Step 3: Welding the Battery Pack to the PCM/BMS

Next, the cells are welded to a PCM (Protection Circuit Module) or a BMS (Battery Management System). This is essentially the “brain” of the pack.

It protects the battery from:

• Overcharging
• Over-discharging
• Short circuits

For low-voltage packs (<20 cells), a PCM with balancing features is often enough. For high-voltage packs (>20 cells), a BMS is essential, since it continuously monitors the cells to maintain safe operation.

Step 4: Insulating the Semi-Finished Pack

At this stage, insulation materials are applied and voltage sensing wires, output cables, and positive/negative terminals are secured.

⚠️ Care must be taken to avoid compressing or stacking wires, especially voltage sampling wires. Pressure on these can damage insulation and increase the risk of short circuits. Proper insulation ensures safe handling before the final assembly.

Step 5: Testing the Semi-Finished Pack

The semi-assembled battery is now tested for quality and performance. Typical tests include:

• Charge/discharge cycles
• Internal resistance checks
• Capacity measurements
• Safety tests (overcharge, over-discharge, short-circuit, overcurrent)

Depending on the application, additional stress tests may also be performed—such as high/low-temperature testing, nail penetration, drop resistance, and salt spray corrosion.

During these tests, technicians closely monitor how the battery responds. For example:

• In overcharge tests, they verify if the BMS safely handles high voltage.
• In short-circuit tests, they check whether the system can manage sudden current surges.
• In overcurrent tests, they see how well the BMS manages strong pulse currents.

Only after passing all tests does the pack move on to packaging.

Step 6: Battery Pack Packaging

The packaging method depends on the pack design.

• For PVC-wrapped packs, a heat-shrinking machine is used.
• For metal case packs, external casing is assembled.

Throughout the process, the battery must be handled carefully to avoid impact or compression. Proper insulation of wires is also crucial to prevent shorts..

 

Step 7: Final Pack Testing

In the last stage, the completed pack is tested on a comprehensive battery tester. Parameters such as:

• Total voltage
• Internal resistance
• Charge/discharge performance

are verified. If necessary, additional protection tests (short-circuit, overcurrent) are also carried out to ensure full safety.

 

Final Thoughts

After these steps, the lithium battery pack is fully assembled and ready for use. While the process may sound complicated, following each step carefully ensures a pack that is both safe and reliable, tailored to its intended application.