Common battery packs generally use three cooling methods: liquid cooling, air cooling, and natural cooling. Battery cells are quite sensitive to temperature, with an optimal operating temperature generally between 15 and 35°C. Temperature variations cause varying degrees of capacity reduction in lithium batteries; specifically, at -10°C, usable capacity is 70%; at 0°C, it's 85%; and at 25°C, it's 100%. Among these three main cooling methods, natural cooling is slow, inefficient, and difficult to control cell temperature, failing to meet the heat dissipation requirements of current energy storage systems composed of large-capacity cells. Therefore, the current energy storage market primarily uses liquid-cooled battery packs.
Before delving into battery packs, let's understand the concepts of cell, battery pack, and battery stack.
First, let's introduce the cell: A cell is essentially a single battery cell, the core component of a battery, and the most basic element in battery packs and stacks. It typically provides a voltage between 3V and 4V.
A battery cell is a sealed, independent unit connecting positive and negative electrodes, typically made from materials such as lithium-ion, nickel-metal hydride, or lead-acid batteries. A cell includes a positive electrode, a negative electrode, and separators to ensure no direct electronic contact between the positive and negative electrodes.
Lithium-ion battery cells are mainly packaged in two categories based on their casing: hard-case and pouch. Hard-case packaging primarily uses steel and aluminum casings, and is further divided into cylindrical and prismatic shapes based on the arrangement of the positive and negative electrodes. Pouch packaging mainly uses aluminum-plastic film. With continuous advancements in packaging technology and processes, cylindrical, prismatic, and pouch packaging methods have gradually formed a three-way balance.
These three packaging methods correspond to three winding methods for lithium-ion batteries: cylindrical lithium-ion batteries correspond to cylindrical winding, prismatic lithium-ion batteries to prismatic winding, and pouch lithium-ion batteries to prismatic stacked winding.
Among them, cylindrical batteries have the advantages of higher production efficiency and lower production cost, while their disadvantages mainly include relatively lower single-cell energy density and safety. Soft-pack batteries use aluminum-plastic film packaging, have good safety, are lighter than steel-cased and aluminum-cased batteries, have higher specific energy, lower internal resistance, and longer cycle life. However, due to the large number of models, low automation, low production efficiency, high cost, heavy reliance on imported high-end aluminum-plastic film, and poor consistency, they are less suitable for these products.
Battery packs: Composed of multiple battery cells, forming a single physical module that provides higher voltage and capacity (e.g., a battery module using four cells connected in series to provide a nominal 12V voltage, or multiple cells connected in parallel to provide greater capacity).
Battery packs: Generally composed of multiple battery packs, and also equipped with a battery management system (BMS), etc., which is the final product provided to the user by the battery manufacturer. This is what people commonly refer to as lithium batteries.
The lithium battery pack process is the process of packaging, assembling, and testing lithium batteries, and is an indispensable part of lithium battery manufacturing. Its importance lies in the fact that through the pack process, battery cells, protection boards, circuits, and other components can be assembled together to form a complete lithium battery product, thereby ensuring the safety, reliability, and performance stability of the lithium battery.
A pack includes the battery pack, busbar, flexible connection, protection board, outer packaging, output (including connectors), and auxiliary materials such as cardboard and plastic brackets. The battery pack assembly process is a crucial step in battery pack production, directly impacting the safety and reliability of lithium batteries. The frequent incidents of lithium battery fires and explosions causing injuries in recent years are largely related to inadequate battery pack design, failing to ensure timely handling and early warning of individual lithium cells or overheating of the battery pack casing.
PACK Requirements and Characteristics:
① Battery packs require high consistency in battery characteristics (capacity, internal resistance, voltage, discharge curve, lifespan).
② The cycle life of a battery pack is lower than that of a single cell.
③ It must be used under defined conditions (including charging and discharging current, charging method, temperature, etc.).
④ After the lithium battery pack is assembled, the battery voltage and capacity are significantly increased, requiring protection through charging equalization, temperature, voltage, and overcurrent monitoring.
⑤ The battery pack must meet the designed voltage and capacity requirements.
PACK Methods:
① Series-Parallel Assembly: Batteries are assembled from individual cells connected in parallel and series. Parallel connection increases capacity without changing voltage, while series connection doubles the voltage without changing capacity. For example, a 3.6V/10Ah battery is composed of 5 N18650/2Ah cells connected in parallel. Parallel connection by series connection: Differences in internal resistance and uneven heat dissipation in parallel connection can affect the cycle life of the batteries. However, if a single cell fails, it automatically disconnects, and apart from a capacity reduction, it does not affect the use of the parallel connection. Parallel connection processes are more stringent. A short circuit in a single cell in a parallel connection can cause a very large current in the parallel circuit, which is usually prevented by adding fuse protection technology. Series connection by parallel connection: The batteries are first connected in series according to the overall capacity of the battery pack. For example, if 1/3 of the total capacity is connected in parallel, it is then connected in parallel last, reducing the probability of failure in large-capacity battery packs.
② Cell Requirements: Select cells according to your design requirements. Batteries connected in parallel and series should be of the same type and model, with differences in capacity, internal resistance, and voltage not exceeding 2%. Generally, after combining batteries in parallel and series, the capacity loss is 2%-5%, and the more batteries there are, the greater the capacity loss. Whether using pouch cells or cylindrical cells, multiple cells need to be connected in series. Poor consistency affects battery capacity; the lowest capacity cell in a group determines the overall capacity of the group. High-current discharge performance is required. The starting current of a motor is three times its normal operating current; high-current discharge improves motor performance. Good heat dissipation is also essential. With a large number of cells, the temperature rise inside the battery pack is difficult to dissipate, resulting in uneven temperature distribution and inconsistent discharge characteristics, ultimately leading to performance degradation. High-level manufacturing processes are required. Batteries must withstand vibrations and impacts from bumpy roads. High standards are placed on manufacturing processes, especially spot welding. Testing is performed after welding to prevent poor soldering or detachment.
③ PACK Process: Battery packing is achieved in two ways: one is through laser welding, ultrasonic welding, or pulse welding, commonly used methods with good reliability but difficult replacement; the other is through elastic metal contact, which eliminates the need for welding and facilitates battery replacement, but may lead to poor contact.
Battery Pack Manufacturing Process
01. Cell Loading and Sorting
Cell Inspection: In the energy storage battery pack process, the first step is cell inspection. Cells are the core components of energy storage batteries, and their performance directly affects the performance and lifespan of the energy storage battery pack. Therefore, cell inspection is a crucial step. The first step on the production line is rigorous testing of the cells, including parameters such as voltage, internal resistance, and capacity. Qualified cells are sorted according to performance parameters to ensure consistency within each pack.
Sorting: Before assembling cells into battery packs, they need to be sorted, i.e., classified according to different parameters. This ensures that different cells, when combined into a group, have similar performance, thereby improving the overall performance and lifespan of the battery pack.
02. Cell Stacking
The cells that have passed initial screening and testing are transported to the stacking station. Here, workers sort the cells and fix them using separators and end plates.
03. Module Deployment
The stacked cell modules are transported to the PACK assembly line, ready for the next process. The modules undergo rigorous inspection to ensure they meet production requirements.
04. Pre-Soldering CCD Addressing
Before soldering, a CCD system is used for addressing. This step ensures the correct arrangement of the cells, providing precise positioning for subsequent soldering.
05. Soldering and Inspection
Next, soldering is performed to ensure a strong and reliable connection between the cells. After soldering, comprehensive testing is conducted, including tests for parameters such as capacitance, voltage, and current, to ensure the module's quality and performance.
06. Protection and Installation
To protect individual cells from overcharging or over-discharging, protection boards are installed. Connectors and cables are then connected to the module for connection to other systems.
07. Module Testing and Decommissioning
Finally, the module undergoes an EOL test to ensure its quality and performance meet requirements. After passing the test, the module is decommissioned, ready for the next process or packaging and shipping.
Battery Pack Testing Content
After the battery pack completes the enclosure testing, the following steps are typically performed:
1. Functional Testing: Ensures the battery pack functions correctly after assembly into the enclosure. This includes charging and discharging tests to check if the battery pack's voltage, capacity, energy density, and other parameters meet requirements.
2. Safety Testing: Tests the battery pack's safety performance, including short-circuit testing, overcurrent protection testing, and temperature testing, to ensure the battery pack will not cause fires, explosions, or other safety accidents during normal use.
3. Environmental Adaptability Testing: Places the battery pack under different temperature, humidity, and vibration conditions to test its stability and reliability under various environmental conditions.
4. Reliability Testing: Conducts long-term charge-discharge cycle tests on the battery pack to simulate lifespan and reliability under real-world usage conditions.
5. Overall Performance Evaluation: Comprehensively evaluates the above test results to determine if the battery pack meets design requirements and makes necessary adjustments and improvements.
6. Pre-Market Preparation: If the battery pack passes all tests and evaluations, it can be prepared for market release. This includes product certification, preparation of product manuals, and packaging design.
