Introduction of battery module

The battery module can be understood as the intermediate product between the battery cell and the pack formed by the combination of lithium-ion battery cells in series and parallel and the installation of single battery monitoring and management device. Its structure must support, fix and protect the core, which can be summarized into three major items: mechanical strength, electrical performance, thermal performance and fault handling capacity. Whether it can fix the position of the cell and protect it from the deformation of damaging performance, how to meet the requirements of current carrying performance, how to meet the control of the cell temperature, whether it can cut off the power in case of serious abnormality, whether it can avoid the transmission of thermal runaway and so on, will be the criteria to judge the battery module. For high performance battery modules, the solution of thermal management has turned to liquid cooling or phase change materials.

Among the three common packaging forms of lithium battery, the single energy density of the soft pack battery is the easiest to be high. However, at the level of module design, the task of considering the overall safety of the product is the most important. It can be said that the activity of a part of the cell is transferred to the module structure.

Main components of the module

There is a big gap in the design and selection of soft pack batteries. The above one is a more typical form. Its basic components include: module control (commonly referred to as BMS slave board), battery unit, conductive connector, plastic frame, cold plate, cooling pipe, pressing plates at both ends and a set of fasteners that combine these components. In addition to the function of gathering the single electric core and providing a certain pressure, the pressing plate at both ends often designs the fixed structure of the module in the pack.

Structural design

Structural design requirements. Reliable structure: anti-seismic, dynamic and anti fatigue; controllable process: no over welding and lack of welding, ensuring 100% of the electric core without damage; low cost: low automation cost of pack production line, including production equipment and production loss; easy to split: battery pack is easy to maintain and repair, low cost, and the electric core can be echeloned and used well; To achieve the necessary heat transfer isolation and avoid the rapid spread of runaway heat, this step can also be considered in the package design.

It is understood that at present, the module group efficiency of cylindrical electric core in the industry is about 87%, and the system group efficiency is about 65%; the module group efficiency of soft pack electric core is about 85%, and the system group efficiency is about 60%; the module group efficiency of square electric core is about 89%, and the system group efficiency is about 70%. The energy density of the single body of the soft clad core is higher than that of the cylinder and the square, but it has higher requirements for the module design and is not easy to control the safety, which are all problems that need to be solved by the structural design.

General module optimization approach. Improving space utilization is also an important way to optimize the module. Power battery pack enterprises can improve the design of module and thermal management system, reduce the cell spacing, so as to improve the utilization of space in the battery box. Another solution is to use new materials. For example, the bus bar in the power battery system (bus in parallel circuit, generally made of copper plate) is replaced by aluminum, and the module fixings are replaced by high-strength steel and aluminum, which can also reduce the weight of the power battery.

Thermal design

The physical structure of the flexible core determines that it is not easy to explode. Generally, only when the pressure that the shell can bear is high enough, it can explode. When the internal pressure of the flexible core is large, it will start to release pressure and leakage from the edge of the aluminum plastic film. At the same time, the soft clad core is also one of several core structures, with the best heat dissipation.

Nissan's leaf, the famous representative of the soft pack battery, has a fully sealed module structure, without considering heat dissipation, that is, without heat dissipation. And the capacity of leaf feedback frequently in the market decays too fast, which is also related to this thermal management. Obviously, with the pursuit of high-performance electric vehicles, it is necessary to have an active thermal management structure for the flexible core.

At present, the mainstream cooling methods have been transformed into liquid cooling and phase change material cooling. Phase change material cooling can be used together with liquid cooling, or alone in the environment is not too bad. In addition, there is also a process which is still widely used in China at present. Here is filled with a thermal conductivity far greater than that of air. The heat released by the telecommunication is transferred to the module shell by the heat conductive adhesive, and then further distributed to the environment. In this way, it is impossible to replace the electric core by itself again, but to a certain extent, it also prevents the spread of thermal runaway.

Liquid cooling: in the picture of module composition described above, the cold plate and liquid cooling pipe are just the components of liquid cooling system. The module is made up of stacked electric cores, and there is a liquid cooling plate arranged at intervals between the electric cores, which ensures that each electric core has a large surface contact with the liquid cooling plate. Of course, it is not easy for the soft clad core to make the liquid cooling technology mature. It must consider the fixing, sealing, insulation and so on of the liquid cooling plate.

electrical design

Electrical design, including low-voltage and high-voltage two parts.

Low voltage design generally needs to consider several functions. Through the signal acquisition harness, the battery voltage and temperature information is collected to the module slave control board or the so-called module controller installed on the module; the module controller is generally designed with equalization function (active equalization or passive equalization or both); a small number of relay on-off control functions can be designed on the slave control board or the module controller; The module controller and the main control board are connected through can communication to transmit the module information.

High voltage design is mainly the series and parallel connection between the electric core and the electric core, as well as the external part of the module, to design the connection and conductive mode between the module and the module. Generally, the series connection mode is only considered between the modules. These high-voltage connections need to meet two requirements: first, the conductive parts and contact resistance between the cores should be evenly distributed, otherwise the detection of single voltage will be disturbed; second, the resistance should be small enough to avoid the waste of electric energy in the transmission path.

safety design

Safety design can be divided into three backward requirements: good design to ensure no accidents; if not, it is better to give early warning to people in advance to reflect the time; if faults have occurred, the design goal is to prevent the accident from spreading too fast.

The first goal is to achieve a reasonable layout, a good cooling system, and a reliable structural design; the second goal is to make the sensor more widely distributed to every possible fault point, and comprehensively detect the voltage and temperature