Discussion on the overall design project of battery

一、Module overall design features

The battery module can be understood as an intermediate product between the battery cell and the battery pack formed by the combination of the lithium-ion battery cell in series and parallel, and the voltage and temperature monitoring and management device of the single battery. Its structure must support, fix and protect the cell, and the design requirements need to meet the requirements of mechanical strength, electrical performance, heat dissipation performance and fault handling ability. Whether it can fully fix the cell position and protect it from deformation that damages performance, how to meet the requirements of current carrying performance, how to meet the control of the temperature of the cell, whether to power off when encountering serious abnormalities, whether to avoid thermal runaway propagation, etc., will be the criteria for judging the merits of the battery module.
 

Figure 1： Square hard shell power battery pack

 

Figure 2： Square soft pack power battery pack

Figure 3: Cylindrical battery pack

二、Electrical performance requirements

● Cell group consistency requirements:

Due to the limitation of the production process, it is impossible to achieve the complete consistency of the parameters of each cell. In the process of series use, the cell with large internal resistance is first discharged, and first fully charged, long-term use, the difference in capacity and voltage of each series cell is becoming more and more obvious. There are eight consistency requirements that need to be considered when selecting cells for modules.
1.Consistent capacity 
2.Consistent voltage 
3.Consistent constant current ratio
4.Consistent power 
5.Consistent internal resistance 
6.Consistent self-discharge rate 
7.Consistent production batch 
8. Consistent discharge platform

● Low voltage design requirements：

The module is composed of a certain number of battery cells in series and parallel, including two parts of low-voltage and high-voltage lines. The low-voltage line shoulders the task of collecting the voltage and temperature signal of the single cell and is equipped with the corresponding balance circuit. Some manufacturers will design a PCB board with fuses to protect the single battery one by one, and the combination of PCB board and fuse protection is also used, once a certain point of failure, the fuse works, the fault battery is disconnected, other batteries work normally, and the safety is high.

Figure 4:  Square hard shell module structure diagram

● High Voltage design requirements：

When the number of cells reaches a certain degree and exceeds the safe voltage of 60V, the high-voltage circuit is formed. The high-voltage connection needs to meet two requirements: first, the distribution of conductors and contact resistance between the cell should be uniform, otherwise the voltage detection of the single cell will be interfered with. Secondly, the resistance should be small enough to avoid the waste of electrical energy on the transmission path. Electrical isolation between high and low voltage lines should also be considered to ensure high voltage safety.

三、Design requirements for mechanical structures

The mechanical structure of the module needs to meet the national standard design requirements, anti-vibration, anti-fatigue. There is no virtual welding between the welding of the battery core, and the case of over-welding, the sealing of the battery pack is good. It is understood that the composition efficiency of modules and battery packs in the industry is as follows

	Group efficiency	Battery pack efficiency
Cylindrical cell	87%	65%
Square cell	89%	68%
Soft cell	85%	65%

Efficiency statistics of different battery groups and battery packs
Improving space utilization is an important way to optimize the module, power battery PACK enterprises can improve the module and thermal management system design, reduce the cell spacing, so as to improve the utilization of the space inside the battery box. Another solution is to use new materials. For example, the bus in the power battery system (the bus in the parallel circuit, generally made of copper plate) is replaced by copper with aluminum, and the module fasteners are replaced by sheet metal materials with high-strength steel and aluminum, which can also reduce the weight of the power battery.

四、 Module thermal design 

At present, the thermal management of power battery systems can be mainly divided into four categories, natural cooling, air cooling, liquid cooling, and direct cooling. Among them, natural cooling is a passive thermal management method, while air cooling, liquid cooling, and direct cooling are active, and the main difference between the three is the difference in the heat transfer medium.

● Natural cooling

Natural cooling there is no additional device for heat transfer. 

● Air cooling

Air cooling uses air as a heat transfer medium. Divided into passive air cooling and active air cooling, passive air cooling refers to the direct use of external air heat transfer cooling. Active air cooling can be considered to heat or cool the external air in order to dissipate or warm the battery.

● Liquid cooling

Liquid cooling uses antifreeze (such as ethylene glycol) as the heat transfer medium. In the scheme, there are generally many different heat exchange circuits, such as VOLT with radiator circuit, air conditioning circuit, PTC circuit, battery management system according to the thermal management strategy for response adjustment and switching. The TESLA Model S has a circuit in series with the motor cooling. When the battery needs to be heated at a low temperature, the motor cooling circuit is in series with the battery cooling circuit, and the motor can heat the battery. When the power battery is at high temperature, the motor cooling circuit and the battery cooling circuit will be adjusted in parallel, and the two cooling systems will dissipate heat independently.

● Direct-cooling

Direct cooling using refrigerant (phase change material) as a heat transfer medium, refrigerant can absorb a lot of heat in the process of liquid phase change, compared with the refrigerant heat transfer efficiency can be increased by more than three times, more quickly take away the heat inside the battery system. Direct cooling was used in the BMW i3.
Battery system thermal management solutions need to consider the consistency of all battery temperatures in addition to the cooling efficiency. The PACK has hundreds or thousands of cells, and the temperature sensor cannot detect every cell. For example, there are hundreds of batteries in a module of Tesla Model S, and only two temperature detection points are arranged. Therefore, the battery needs to be as consistent as possible through thermal management design. And better temperature consistency is the premise of consistent battery power, life, SOC and other performance parameters.

At present, the mainstream cooling method on the market has changed to a combination of liquid cooling and phase change material cooling. Phase change material cooling can be used in conjunction with liquid cooling, or alone in less harsh environmental conditions. In addition, there is a process that is still more widely used in China, and the thermal conductivity adhesive process is applied to the bottom of the battery module. The thermal conductivity of thermal glue is much higher than that of air. The heat emitted by the battery cell is transferred by the thermal conductive adhesive to the module housing, and then further dissipated to the environment.

Summary: 

In the future, major Oems and battery factories will carry out fierce competition in the design and production of modules around performance improvement and cost reduction. Performance needs to meet the requirements of mechanical strength, electrical performance, heat dissipation performance and other three aspects to further enhance the core competitiveness of the product. In terms of cost, in-depth research on the standardization of smart cells is carried out to lay the foundation for further expansion of production capacity, and vehicle flexibility can be achieved through the combination of different kinds of standardized cells, and ultimately a significant reduction in production costs.