About the Energy Storage System - Lithium-ion Battery Pack
Lithium-ion battery pack technology is an important part of the skills of the energy storage industry. Let’s know about some basic knowledge of battery pack together.
1. Definition of battery pack
Lithium-ion battery pack, also known as battery module, is a manufacturing process of lithium-ion batteries, which is the packaging, encapsulation, and assembly. It is formed by connecting multiple lithium-ion single cell groups in parallel and in series, and considering the mechanical strength of the system, thermal management, BMS matching and other issues.
Its important technologies lie in the overall structural design, welding and processing process control, protection level, active thermal management system, etc. If two batteries are connected in series or in parallel to form a specific shape according to customer requirements, it is called battery pack.
2. The composition of the battery pack
The composition of the battery pack mainly includes the single battery module, electrical system, thermal management system, box and BMS.
▶ Battery module
If the battery pack is compared to a human body, then the module is the “heart”, which is responsible for the storage and release of electric energy.
▶ Electrical system
It is mainly composed of connecting copper bars, high-voltage wiring, low-voltage wiring, and electrical guarantee devices. The high-voltage wiring can be regarded as the “aortic blood vessels” of the battery pack, which continuously delivers battery power to the terminal load. While the low-voltage wiring can be regarded as the “neural network” of the battery pack, which transmits detection signals and control signals in real time.
▶ Thermal management system
There are two main thermal management systems: air cooling and liquid cooling.
Liquid cooling can be divided into cold plate liquid cooling and immersion liquid cooling. The thermal management system is equivalent to an air conditioner for the battery pack.
The battery will generate heat in the discharge mode. In order to ensure that the battery works at a reasonable ambient temperature and improve the battery cycle life, it is generally required that the temperature difference of the system is ≤5°C.
It is mainly composed of box, box cover, metal bracket, panel and fixing screws. It can be regarded as the “skeleton” of the battery pack. It plays the role of support, mechanical shock resistance, mechanical vibration and environmental protection.
The Battery management system can be regarded as the “brain” of the battery. It is mainly responsible for measuring parameters such as voltage, current and temperature of the battery, and also has functions such as equalization. And it can transfer data to MES.
3. Features of battery pack
▶ Lithium battery packs require batteries to have a high degree of consistency (capacity, internal resistance, voltage, discharge curve, lifespan).
▶ The cycle life of the battery pack is lower than that of a single battery.
▶ Use under limited conditions (including charging and discharging current, charging method, temperature, etc.).
▶ The battery voltage and capacity of the lithium battery pack are greatly improved after being formed, so it must be protected and monitored for charge equalization, temperature, voltage and overcurrent.
▶ The battery pack must meet the voltage and capacity requirements required by the design.
4. The assembly method of battery pack
▶ Composition in series and parallel
The battery module is composed of single cells connected in parallel or in series. Parallel connection increases the capacity, but the voltage remains the same. The series connection makes the voltage double, but the capacity remains the same.
① 15 batteries with a voltage of 3.2V connected in series are 48V, which is series and boost.
② Two batteries with a capacity of 50Ah are connected in parallel, and there is 100Ah, which is parallel expansion.
▶ Battery requirements
Select the corresponding batteries according to your own design requirements. The batteries used for parallel connection and series connection should be of the same type and model, and the difference in capacity, internal resistance and voltage value should not exceed 2%. Whether it is a flexible package battery or a cylindrical battery, multiple string combinations are required.
▶ Process of assembly
The assembly of the battery is realized in two ways.
One is by laser welding, ultrasonic welding or pulse welding, which is a commonly used welding method.
- Advantage:good reliability.
- Disadvantage: not easy to replace.
The second is to contact through the elastic metal sheet.
- Advantages: no welding required, easy battery replacement.
- Disadvantages: possibility of poor contact.
Considering the production yield, efficiency and the internal resistance of the connection point, laser welding is currently the first choice of many battery manufacturers.
5. The technical parameters of the battery pack
▶ Combination method - 1P24S
- S stands for series cells, P stands for parallel cells.
- 1P24S means: 24 strings and 1 parallel.
- After series connection, the voltage doubles, and the rated voltage is 3.2*24=76.8V.
▶ Rated capacity - 280Ah
The rated capacity of the battery refers to the capacity that the battery can continue to work for a long time under the rated working conditions.
The rated capacity C of the battery, in ampere hours (Ah), is the product of the discharge current (A) and the discharge time in hours (h). So 280Ah means discharge at a maximum rate of 0.5C, which can last for 2 hours.
▶ Rated Energy - 21.504kWh
Rated energy (Wh) = nominal capacity
(Ah) * nominal voltage (V), so the total energy that a battery can release is related to capacity and voltage.