What is The Battery Formation And Capacity Grading Process?-xmacey.com

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What is The Battery Formation And Capacity Grading Process?

October 27 , 2025

What is the battery formation and capacity grading process?

In the lithium battery manufacturing process, formation and capacity grading are critical steps that determine battery performance and lifespan. The principle of capacity grading serves as the theoretical foundation, while formation and grading procedures ensure product consistency and reliability. Below is a detailed overview of both concepts and their operational methods.

1. What is Formation and Capacity Grading?

Formation:

Refers to a series of technological treatments applied during the first charging of the battery to stabilize its performance, including low-current charge and discharge and constant temperature resting, etc.

Capacity Grading:

Simply understood as capacity classification and performance sorting, in which batteries are screened and graded according to their performance parameters.

2. Lithium Battery Formation Process and Principle

During the initial charge–discharge process of a lithium-ion battery, non-protonic solvents used as the electrolyte unavoidably react at the interface between the electrode and the electrolyte, forming a passivation film on the electrode surface. This film is known as an electron-insulating layer or Solid Electrolyte Interphase (SEI) film.

According to reports, the passivation film is composed of Li₂O, LiF, LiCl, Li₂CO₃, LiCO₂–R, alcoholates, and non-conductive polymers. It has a multilayer structure — the side near the electrolyte is porous, while the side near the electrode is dense.

The formation of the SEI film has a crucial impact on the performance of electrode materials.
On one hand, the formation of the SEI film consumes part of the lithium ions, increasing the irreversible capacity of the first charge–discharge and reducing the charge–discharge efficiency of the electrode material.
On the other hand, since the SEI film is insoluble in organic solvents, it can remain stable in the organic electrolyte solution. Solvent molecules cannot penetrate the SEI layer, effectively preventing solvent co-intercalation and avoiding damage to the electrode materials. This significantly improves the cycling performance and service life of the electrode.

The quality of the formation process determines the quality of the SEI film, directly affecting the battery's cycle life, stability, self-discharge rate, safety, and other electrochemical characteristics. Therefore, battery manufacturers develop suitable formation and grading processes and parameters based on their production practices to ensure that batteries achieve maximum capacity, best stability, and longest lifespan.

Battery formation requires long charging and discharging cycles. During charging, it is necessary to precisely control the charging and discharging voltage and current, as well as the pulse waveform, to prevent an increase in SEI film impedance, which would negatively impact the high-rate discharge performance of lithium-ion batteries. Accurate control also improves the efficiency of the formation process.

For example, in LiFePO₄ systems, when the charging voltage exceeds 3.7 V, it may damage the crystal structure of LiFePO₄, thereby reducing the battery's cycling performance.

3. Lithium Battery Capacity Grading Steps

Battery capacity grading is performed using a battery formation and grading system(since formation and grading share similar principles, both functions are integrated into one cabinet). The formation and grading cabinet essentially functions like a multi-channel charger, capable of charging and discharging a large number of cells simultaneously.

During the grading process, data from each test point are collected and managed by a computer system to analyze parameters such as capacity and internal resistance, determining the quality level of each cell — this process is called capacity grading.

After the initial capacity grading, batteries need to rest for a period of time, generally no less than 15 days. During this period, inherent quality issues may become apparent, such as excessive self-discharge and increased internal resistance. After the rest period, batteries are retested and regraded, and those that do not meet capacity standards or have quality issues are eliminated.

Another purpose of capacity grading is to classify and group batteries. This involves selecting cells with similar internal resistance and capacity for combination. Only cells with similar performance are combined into a battery pack. For example, power battery packs often require tens to thousands of cells to meet the energy requirements of electric vehicles. Due to the complexity of the system, battery pack behavior is unique, and pack performance cannot be achieved by simply adding or subtracting individual cells.

Taking the common battery packs composed of series and parallel connections as an example, ideally, all single cells in the battery pack should be completely consistent. However, in reality, even single cells produced in the same batch still have performance differences (including factors such as capacity and internal resistance). In the lithium battery pack assembly line, we also need to use battery sorting machines to sort the battery voltage and internal resistance again. Although screening is performed before the battery pack is assembled, it is still impossible to guarantee 100% consistency in the performance of all batteries. In addition, due to the volume of the battery pack, the heat dissipation characteristics of different parts also vary greatly. Therefore, there is a large temperature gradient in the temperature distribution of the battery pack. The above factors will cause the batteries in the battery pack to decay at different rates during use. In this case, the available capacity of the battery pack will decrease (limited by the minimum capacity of the series-connected cells in the battery pack). On the other hand, it may also lead to reduced safety of the battery pack. Studies have shown that even if the cycle life of a single cell can reach more than 1,000 times, when forming a battery pack, if there is no protection of battery balancer, the cycle life of the battery pack may be less than 200 times. Therefore, for a battery pack, the consistency of the single cell is a very important parameter.

4. Precision Requirements for Formation and Grading

In order to improve the battery's electrochemical properties, such as cycle life, stability, self-discharge, and safety, the consistency of lithium batteries must be strictly controlled or the battery grade must be accurately assessed. Therefore, there are very high requirements for the current and voltage measurement accuracy of formation and capacity grading equipment. Acey New Energy has over 15 years of experience in the battery equipment industry, mainly providing high-end equipment and one-stop solutions for lithium battery laboratory research and lithium battery pack assembly.

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