Process knowledge of lithium battery pack

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PACK Introduction
Lithium batteries are widely used, ranging from civilian digital and communication products to industrial equipment to military power supplies, etc. They are all used in batches. Different products require different voltages and capacities. Therefore, lithium-ion batteries are used in many cases in series and parallel. The application battery formed by protecting the circuit, casing and output is called PACK.
PACK can be made of a single cell, such as mobile phone batteries, digital camera batteries, MP3, MP4 batteries, etc., or it can be a combination of multiple cells in series and parallel, such as laptop batteries, power tool batteries, electric vehicle batteries, energy storage battery, etc.

1. PACK composition
PACK includes cells, BMS, outer packaging or shell, output (including connectors), key switch, power indicator, and auxiliary materials such as EVA, Barley paper, and plastic holders to form a PACK. The external characteristics of PACK are determined by the application, and there are many types of PACK.

2.Use of each material

3. Features of PACK
★ There are complete functions, which can be directly applied.
★ Variety of types. There are multiple PACKs that can implement the same application requirement.
★ The battery PACK requires the cell to have a high degree of consistency (capacity, internal resistance, voltage, discharge curve, life).
★ The cycle life of the battery PACK is lower than the cycle life of a single battery.
★ Use under limited conditions (including charging, discharging current, charging method, temperature, humidity conditions, vibration, stress, etc.)
★The lithium battery PACK protection board requires a charge equalization function.
★High-voltage, high-current battery packs (such as electric vehicle batteries, energy storage systems) require communication buses such as battery management system (BMS), CAN, and RS485.
★The battery PACK has higher requirements on the charger, and some require communication with the BMS. The purpose is to make each battery work normally, fully utilize the energy stored in the battery, and ensure safe and reliable use.

4. The design of PACK
★ Fully understand the usage requirements, such as the application environment (temperature, humidity, vibration, salt spray, etc.), usage time, charging, discharging methods and electrical parameters, output methods, life requirements, etc.
★ Select qualified cells and protection circuit boards according to usage requirements.
★ Meet the requirements of size and weight.
★ The packaging is reliable and meets the requirements.
★ The production process is simplified.
★ Program optimization.
★ Minimized cost.
★ Detection is easy to implement.

5. Precautions for use
★ Do not put it into fire or use it close to a heat source.
★ Do not use metal to directly connect the positive and negative electrodes of the output together.
★ Do not use beyond the battery temperature range.
★ Do not squeeze the battery with force.
★ Use a special charger or charge it in the correct way.
★ Please recharge the battery every three months when the battery is on hold. and place at storage temperature.

Introduction of important components
1. Battery module
★ Composition: The battery consists of single cells connected in series and parallel.
Parallel connection increases the capacity, the voltage remains unchanged, series connection increases the voltage, the capacity remains unchanged, such as a 3.6V/10Ah battery is composed of a single 18650/2Ah through 5 parallels, a 36V/2Ah battery can be composed of a single 18650/2Ah through 10 series, 36V /10Ah battery consists of a single 18650/2Ah through 5 parallel 10 strings.
★ Combination requirements: parallel and series cells require the same type and model, and the difference in capacity, internal resistance and voltage value is not greater than a certain value. The essence is the consistency (dynamic consistency) of the battery charge-discharge curve. But it is difficult to achieve. In general, after the cells are combined in parallel and series, the capacity loss is 2% to 5%. The more cells there are, the greater the capacity loss.
★ Realization of combination: The combination of cells is realized in two ways. One is through nickel strip spot welding or laser welding or ultrasonic welding, which is a common method. The advantage is that the reliability is good, but it is not easy to replace. The second is to contact through elastic metal sheets. The advantage is that no welding is required, and the cell is relatively easy to replace. The disadvantage is that it may lead to poor contact.
★ Requirements: The battery pack must meet the user’s working time requirements, environmental requirements, vibration requirements, charging requirements, life requirements, etc. The battery module cannot be used alone (overcharge, overdischarge, and overcurrent will damage the cell), it needs to be equipped with a BMS before it can be used.

2. BMS
★ Composition: The BMS is composed of IC, MOSFET, transistor, resistor and capacitor, PCB and heat sink.
★ Function: Protect the battery to work under normal conditions. Prevent the battery from failure due to overcharge, overdischarge, short circuit, etc. and the danger of smoke, fire and explosion.
★ Specific functions:
A. Overcharge protection, the overcharge protection voltage of batteries of different systems is different. In general, the overcharge protection voltage of LFP is 3.65V~3.85V, and the overcharge protection voltage of LCO, LMO, NCM material lithium battery is 4.2V~4.35V .
B. Over-discharge protection, the over-discharge protection voltage of LFP is 2~2.5V, and the other three lithium batteries are 2.3V~2.9V.
C. short circuit protection. It is determined by the parameters of IC and MOSFET.
D. overcharge protection recovery voltage. IC has this function, generally the recovery voltage is lower than the protection voltage 0.1~0.2V.
E.Over-discharge protection recovery voltage. Some ICs do not have this function. After over-discharge, it needs to be restored by charging.
F.Short circuit protection delay time. Usually ten milliseconds.
G.Balance function. Some applications require equalization. Due to the large discharge current of the battery, the balance is not easy to achieve. At present, it is mostly charge balance. The purpose of balance is to ensure that each battery can be fully charged, prolong the service life, and maximize the role of the battery.
★ Use of BMS
A. The BMS is connected close to the battery, and the lithium battery cannot be used separately from the BMS.
B. Pay attention to effective isolation between the BMS and the battery for insulation and heat insulation.
C. When the multi-section protection board is installed and connected to the battery, connect it in the order of low level first and then high level. That is, connect B- first, then B1, B2…B9, and then B+. See below. When removing the protective plate, the installation sequence is reversed.

★ Design of BMS
The following points should be considered in the design of BMS:
A. The design should ensure reliability and working stability, and at least 30% technical allowance should be reserved for overcurrent parameters.
B. Design the product to be easy to test.
C. The design should be convenient for production and debugging, and the process should be simplified.
D. The product should be easy to install.
E. Design to meet dimensional requirements.
F. The design should be convenient to purchase the device, and the device should not be too special.
G. Design with cost in mind.
H. The design should fully consider the heat dissipation and the placement of the heat dissipation components.
I. The design should consider environmental protection requirements.

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