A Brief Introduction of Battery Pack

Ⅰ. What is a battery pack?

A battery pack is a group of electrical circuits composed of batteries used to provide electrical energy. A battery is typically an organic power source that stores electrical charge and electrical energy. A battery pack can consist of multiple batteries, each with its own voltage and capacity. Battery packs can be combined in series and parallel to achieve higher voltage or more power output. It is usually connected by one or more chargers to supply power to devices that require power.

Ⅱ. The basic concept of battery

1. Battery

It consists of more than one battery cells connected in parallel or in series. It is housed in a physically separate battery case with separate positive and negative outputs. The 12V or 24V starter battery commonly used in internal combustion engine vehicles is composed of 6 or 12 2V lead-acid battery cells connected in series.

2. Battery Cell

It is the basic device and basic unit that directly converts chemical energy into electrical energy, and is the basic component of a battery, including electrodes, separators, electrolytes, casings, etc.

3. Battery Module

For battery packs that do not contain complete battery management functions, we usually call them battery modules.

4. Battery Pack

It is a component that stores electric energy or outputs electric energy externally, which is composed of multiple batteries connected in series or in parallel. Usually, the current meaning of the battery pack also includes power battery management system, battery box and other components.

Ⅲ. The composition of the battery pack

The structure and performance of the battery pack directly affect the use and safety of electric equipment. It mainly includes electrical system, thermal management system, mechanism system, battery module and BMS.

1. Electrical system: It is mainly composed of high-voltage jumpers or high-voltage wiring harnesses, low-voltage wiring harnesses and relays. The high-voltage wiring harness can be regarded as the "aortic vessel" of the battery pack, which continuously transmits the power of the heart of the power battery system to various required components, and the low-voltage wiring harness can be regarded as the "neural network" of the battery pack, which transmits detection signals and control signal.

2. Thermal management system: There are four main types of thermal management systems: air cooling, water cooling, liquid cooling and phase change materials. Taking the water-cooling system as an example, the thermal management system is mainly composed of cooling plates, cooling water pipes, heat insulation pads and heat conduction pads. The thermal management system is equivalent to assembling an air conditioner for the battery.

3. Mechanism system: It is mainly composed of the battery pack upper cover, tray, various metal brackets, end plates and bolts. It can be regarded as the "skeleton" of the battery pack, which can support, resist mechanical shock, mechanical vibration and environment protection (waterproof and dustproof).

4. Battery module: If the battery pack is compared to a human body, then the module is the "heart", which is responsible for storing and releasing energy to provide power for the car.

5. Battery management system (BMS): It can be seen as the "brain" of the battery. It is a system that monitors and controls the state of the battery to make the battery work more safely and reasonably. It has the function of detecting battery voltage, current, temperature and other parameters, and can detect abnormal battery status in time and take measures. BMS guarantees the safety of the battery pack, prolongs the battery lifespan and improves the reliability of electric vehicles.

(1) Composition: BMS is mainly composed of CMU and BMU.

① Cell Monitor Unit (CMU)

It is responsible for measuring parameters such as voltage, current and temperature of the battery, as well as functions such as equalization. After measuring these data, the CMU transmits the data to the BMU through the battery "neural network" mentioned above.

② Battery Management Unit (BMU)

It is responsible for evaluating the data transmitted by the CMU. If the data is abnormal, it will protect the battery, send a request to reduce the current or cut off the charging and discharging path, so as to prevent the battery from exceeding the permitted use conditions. At the same time, it also manages the power and temperature of the battery. According to the previously designed control strategy, the BMU judges the parameters and states that need to be warned, and sends the warning to the vehicle controller, and finally conveys it to the driver.

(2) Function: BMS mainly covers the following functions:

① Balance between single cells

The battery management system realizes the balanced charging of single batteries through the core component equalizer, so that each battery in the battery pack can reach a balanced and consistent state. However, domestic technology in this area is not mature enough.

② Battery working status monitoring

It monitors or calculates parameters such as battery voltage, temperature, working current and power in real time, and judges the state of the battery based on these parameters to prevent overcharging or overdischarging.

③ Battery charge and discharge management

It manages the charging or discharging of the battery according to relevant parameters such as environmental status and battery status, so as to set the optimal charging or discharging curve, such as charging current, charging upper limit voltage value and discharging lower limit voltage value, etc.

Ⅳ. The characteristics of the battery pack

1. Long lifespan: Since multiple battery cells in the battery pack are connected in parallel, when one or several battery cells fail, other battery cells can continue to work, thereby prolonging the lifespan of the battery pack.

2. Simple maintenance: The maintenance of the battery pack is relatively simple. Since multiple battery cells are connected in parallel, if one of the battery cells needs to be replaced or maintained, we only need to replace or maintain one of them instead of replacing or maintaining all the battery cells.

3. High price: The price of the battery pack is relatively high. Since multiple battery cells need to be connected in parallel, we need more materials and processes, resulting in increased cost.

4. High energy density: The energy density of a battery pack refers to the energy that can be stored by a battery per unit volume or unit weight. Because the energy density of a battery pack is directly related to the energy density of a single battery, generally speaking, the energy density of a battery pack is higher than that of a single battery.

5. Stable power supply: the battery pack can provide stable voltage and current. Because the voltage and current between each battery cell are complementary to each other, it can reduce the fluctuation of supply voltage and current.

Ⅴ. Battery pack process

The battery pack is the core energy source of new energy vehicles, providing driving power for the entire vehicle. As the core component of new energy vehicles, its quality directly determines the performance of the vehicle. Lithium battery manufacturing equipment is generally divided into three types: front-end equipment, middle-end equipment and back-end equipment. Its equipment precision and automation level will directly affect the production efficiency and consistency of products.

Although the types of batteries and modules are different, the composition and process flow of the battery pack are roughly the same. 

Ⅵ. The advantages and disadvantages of the battery pack

1. Advantages

(1) At the end of product life, batteries can be disassembled and recycled individually, thereby reducing the total amount of hazardous waste.

(2) Flexibility in design and implementation enables the use of cheaper high-yield cells or combinations of cells.

(3) People can easily plug it into or take it out of the device. This allows multiple battery packs to provide longer runtime, freeing up the device for continued use while the removed battery pack is charged independently.

2. Disadvantages

For the average user, battery packs are easier to repair or change than those non-repairable cells or batteries. However, it is important to note that when servicing a battery pack, we must take safety precautions as it may pose chemical, electrical and fire hazards.

Ⅶ. Lithium battery pack manufacturing process

1. Allocation selection group

The battery allocation and selection group is to classify the batteries according to the ohmic internal resistance, polarization internal resistance, open circuit voltage, rated capacity, charge and discharge efficiency, self-discharge rate and other parameters of the battery, so as to ensure that the batteries with good consistency of battery parameters are divided into the same category. This not only helps to improve the consistency of the internal characteristics of the battery group, but also helps to improve the efficiency of the module and prolong its service life.

A professional lithium battery matching method includes the following steps:

(1) Test the capacity of the battery cell: We will install the battery cell to be divided into three charging and discharging cycles according to the required current on the testing equipment, and then charge the voltage of the fourth single battery cell to the preset range of rated capacity.

(2) Obtain the matching reference benchmark: We record the discharge capacity of the third cell, constant current charging time and constant voltage charging time and other parameters.

(3) Cell capacity sorting: We use the discharge capacity of the cells in the third cycle as the standard, set the lower limit capacity, and select cells larger than the lower limit capacity as qualified cells.

(4) Preliminary grouping of batteries: We use the two parameters of constant current charging time and constant voltage charging time as a benchmark to group batteries with qualified capacity and the same or similar parameters of constant current and constant voltage charging time.

(5) Cell voltage drop test: We place the assembled cells in a set environment for a period of time to measure their voltage drop. After we determine the qualified voltage drop standard, we sort out the qualified cells.

(6) Final assembly of batteries: We select batteries with qualified voltage drop, and use these batteries for final assembly.

2. Lithium battery cell assembly fixture

The cell mounting jig must be assembled in accordance with the order of the positive and negative electrodes of the cell in the SOP of the PE engineer, otherwise it will cause a short circuit of the cell. After setting up the automatic welding machine program, we put the jig battery into it and start automatic spot welding. After the automatic spot welding is completed, the quality needs to be inspected. If there are leaks and explosions, we need to repair welding.

3. Lithium battery pack welding PCM/BMS

The protection module (PCM/BMS/nPCM or PCB) of the lithium battery pack is the core of the lithium battery pack, responsible for protecting the battery from overcharging, overdischarging and short circuit damage, and avoiding explosion, fire and damage to the lithium battery pack. For low-voltage lithium battery packs, we should choose PCM with balancing function. As for the high-voltage lithium battery pack, we should use advanced BMS to monitor the performance of each battery to ensure the safe operation of the battery. The design steps vary from pack to pack, but we all require quality checks, including PCM spot welds to the pack and solder and screw connections. The connection sequence of voltage acquisition lines is B-, B1... to B+ in turn. Weld after unplugging the voltage acquisition line. After the welding is completed, we will check the acquisition line and connect the BMS after confirming that it is correct.

4. Semi-finished insulation

We need to fix and insulate the voltage collection line and output line, and use auxiliary materials such as high-temperature tape, barley paper, epoxy board, and cable ties. At the same time, we need to pay attention to safety, do not put pressure on the battery pack voltage acquisition line or output wire, so as not to cause a short circuit.

5. Semi-finished product test

After the battery pack is installed with BMS, it can be tested for a semi-finished product. Routine tests include: charge and discharge test, internal resistance test, capacity test, overcharge test, over discharge test, short circuit test, overcurrent test. According to special requirements, it may need special lithium battery pack tests such as high temperature and low temperature tests, acupuncture tests, drop tests, and salt spray tests. These tests may be destructive and spot checks are recommended. In testing, we need to pay attention to the endurance of the battery pack. For example, in the overcharge test, whether the BMS can withstand high voltage; in the short circuit test, whether the BMS can withstand instantaneous high voltage and high current; in the overcurrent test, whether the BMS can withstand pulse current, etc.

6. PACK packaging

Before packaging the battery pack, we must first complete the insulation of the signal acquisition line and the positive and negative poles of the battery pack. For the battery pack in PVC packaging, we need to process it through a heat shrinking machine. Ultrasonic-sealed battery packs require an ultrasonic machine. For battery packs with metal outer boxes, we need to do outer box assembly. In all the process, we should handle it with care, avoid collision and extrusion, and at the same time ensure that the wires are insulated to avoid short circuit.

7. Overall test

According to the preset parameters, we connect the battery pack to the overall tester for testing. The main test items include delivery voltage, internal resistance and simple charging and discharging. In addition, overcurrent and short circuit are also optional test items.

8. Packaging and shipping

We need to pack according to the customized foam size, and don't place it randomly. When shipping battery packs, our worst fear is the empty inner packaging. Therefore, we should ensure that there is enough foam inside for cushioning to avoid impact damage during transportation. For the sake of safety, it is recommended to ship the large battery pack PACK on a wooden frame.

Ⅷ. What kind of battery pack box do we need?

As the carrier of the entire power system, the battery pack box generally needs to have the following functions:

1. Thermal protection function

In a limited space, it can reduce or even isolate the impact of the external environment temperature on the battery cells and electrical performance inside the battery box, maintain a reasonable working temperature, thereby maximizing battery performance and prolonging battery lifespan.

2. Protection level

We often say that the dustproof and waterproof performance of IP level is to prevent dust and water from affecting the insulation performance of the battery cells, electrical components, control system circuit boards, etc. inside the battery pack, and corroding metal devices.

3. Structural protection function

It forms a closed and controllable space, which prevents the internal components from being squeezed by external forces. It protects the cells from physical damage, deformation, and internal short circuits in limited shock and vibration environments. Someone proposed a term "lithium-ion battery mechanical integrity", which refers to the nature of the battery to maintain the normal electrochemical function of the battery under mechanical overload conditions. It can be said that the mechanical integrity of lithium-ion batteries is the ultimate goal of the protective function of the battery pack box structure.