Lithium-ion batteries are widely used in numerous applications such as smartphones, electric vehicles, and large-scale energy storage systems. When it comes to battery performance, people often focus on battery materials or chemical systems. However, after the cell assembly is complete, there are two critical manufacturing processes that significantly impact battery quality— formation and grading.
Forming and capacity grading not only determine whether a battery can be properly activated, but also directly affect its capacity, cycle life, consistency, and safety performance. Even with high-quality battery materials and advanced manufacturing processes, improper forming control or insufficient capacity grading accuracy can still lead to increased cell performance variations, compromise battery pack consistency, and even shorten the product’s service life.
This article will provide a detailed explanation of the working principles of battery formation and capacity grading, their critical roles in lithium-ion battery manufacturing, and key factors that enterprises should focus on when selecting formation and grading equipment, helping manufacturers improve product quality and production efficiency.
What Is Battery Formation?
Formation (the process) refers to the first controlled charging and discharging process that lithium-ion cells undergo after assembly. It marks the activation threshold for the cells to transition from a “physical structure” to an “electrochemical system”. During this stage, the electrochemical reactions within the cells are officially initiated for the first time. Manufacturers activate the active substances of the positive and negative electrode materials gradually through precisely set charging and discharging protocols, while in situ generating a nanoscale solid electrolyte interphase membrane (SEI) on the surface of the negative electrode.
The SEI layer plays a crucial role in battery performance. It protects the electrode surface while allowing lithium ions to pass through during charging and discharging. A good SEI layer can enhance capacity retention rate, extend cycle life and improve battery safety.
During the formation process, manufacturers strictly control multiple parameters, including charging current, voltage, temperature, and charging curve. Even minor changes can affect the long-term stability of the battery. Since the conversion process usually requires multiple charging and discharging cycles as well as a period of standing still, it is also one of the most time-consuming steps in the battery manufacturing process.
What Is Battery Grading?
Once formation is complete, battery cells move to the grading process.
Grading evaluates the electrical performance of every individual cell and groups cells with similar characteristics together. Manufacturers typically measure parameters such as:
- Capacity
- Open-circuit voltage (OCV)
- Internal resistance
- Charge and discharge efficiency
- Self-discharge behavior
No two battery cells are exactly identical, even when produced on the same manufacturing line. Small differences between cells can lead to imbalance inside a battery pack, reducing overall efficiency and accelerating aging.By sorting cells with similar performance into the same group, grading helps improve battery pack consistency and overall system reliability.
How the Formation and Grading Process Works
Although production lines vary between manufacturers, the overall process generally follows several key steps.
Cell Inspection
Before formation begins, cells are inspected to verify polarity, insulation, and open-circuit voltage. Any defective cells are removed before entering the production line.
Formation Cycling
By repeatedly using precisely programmed current and voltage curves, the battery cell undergoes controlled charging and discharging cycles. During this stage, the SEI layer forms and the electrochemical system becomes stable.
Rest Period
After being fully charged, the battery will be left to rest for a period of time. Engineers will also monitor the voltage drop to determine whether the electrochemical reaction is stable.
Performance Testing
The battery will undergo additional charging and discharging cycles to measure its capacity, energy efficiency and internal resistance.
Grading and Sorting
Finally, automated systems classify cells into different performance categories so that only well-matched cells are assembled into battery modules and packs.
Why Formation and Grading Are So Important
Formation and grading directly influence both manufacturing quality and long-term battery performance.
Building a Stable Electrochemical Foundation
Formation determines how well the SEI layer develops. A stable SEI reduces side reactions inside the battery, improves efficiency, and slows capacity degradation throughout the battery’s service life.
Improving Cell Matching
Battery packs consist of hundreds or even thousands of individual cells. If one cell performs differently from the others, it can affect charging balance, heat generation, and overall pack lifespan. Accurate grading minimizes these differences.
Increasing Manufacturing Yield
Formation and grading also identify defective cells early in the production process. Cells with abnormal voltage behavior, excessive internal resistance, or poor capacity can be removed before entering module assembly, reducing waste and lowering manufacturing costs.
Supporting Full Traceability
Modern production lines record detailed data throughout formation and grading. This information allows manufacturers to trace the history of every battery cell, identify process deviations, and continuously improve product quality.
Choosing the Right Formation and Grading Solution
As battery production continues to expand, manufacturers require equipment capable of handling both high throughput and high precision.
Several factors should be considered when selecting a formation and grading solution:
- Measurement accuracy to ensure consistent electrical testing.
- Channel density for processing large numbers of cells simultaneously.
- Scalable architecture that supports future production expansion.
- Automation capabilities for integration with robotic production lines.
- Manufacturing Execution System (MES) compatibility for complete production traceability.
- Energy recovery technology, which can significantly reduce electricity consumption during large-scale charge and discharge testing.
The best solution depends on production volume, battery chemistry, and application requirements. While research laboratories may prioritize flexibility, gigafactories often focus on automation, scalability, and operational efficiency.
Industry Trends
The rapid growth of electric vehicles and battery energy storage systems is driving continuous improvements in formation and grading technology.
Today’s manufacturers are increasingly adopting:
- High-density multi-channel testing systems
- AI-assisted quality analysis
- Predictive maintenance
- High-efficiency regenerative power technology
- Fully automated production lines integrated with MES and factory management systems
These technologies help manufacturers improve production efficiency while maintaining increasingly strict quality requirements.
Conclusion
Cell formation and classification are among the most crucial steps in the manufacturing process of lithium-ion batteries. Formation processes activate the batteries and establish their electrochemical foundation, while classification ensures that only cells with similar electrical properties can be assembled into battery packs. As battery production moves towards larger-scale and more automated manufacturing models, advanced charging and classification technologies are of vital importance for enhancing product consistency, reducing manufacturing costs, and ensuring the long-term reliability of batteries.
For manufacturers who are planning new production lines, when evaluating forming and grading equipment, what needs to be considered is no longer just the performance indicators of a single piece of equipment. Instead, the scalability of the system architecture, the data interfaces with the preceding and subsequent processes, and the feasibility of long-term operation should also be taken into account.
