Introduction to battery modules

The battery module can be understood as a lithium ion battery core combined in series and parallel mode, and an intermediate product of a battery core and a pack formed after the unit battery monitoring and management device is installed. Its structure must support, fix and protect the battery core, which can be summarized into three major items: mechanical strength, electrical performance, thermal performance and fault handling capability. Whether it can perfectly fix the position of the battery core and protect it from deformation with lossy performance, how to meet the current-carrying performance requirements, how to control the temperature of the battery core, whether it can be powered off in case of serious abnormality, whether it can avoid thermal runaway Communication, etc., will be the standard for judging the pros and cons of battery modules. For high-performance battery modules, thermal management solutions have turned to liquid-cooled or phase-change materials.

The energy density of the soft pack battery is the easiest to be done in the three common lithium battery packs. However, at the layer of the module design, the task of considering the overall safety of the product is the heaviest. It can be said that a part of the battery core is used. The live transfer was given to the module structure.

Main components of the module

Soft pack battery, the design choice gap is relatively large, the above figure is a typical form, its basic components include: module control please (often said BMS slave board), battery cells, conductive connectors, plastic frame , cold plates, cooling pipes, pressure plates at both ends, and a set of fasteners that combine these components together. In addition to the function of collecting a single cell and providing a certain pressure, the press plates at both ends are also designed with a fixed structure of the module in the pack.

Structural design

Structural design requirements, reliable structure: anti-vibration and anti-fatigue; process controllable: no over-welding, no-weld, ensuring 100% no damage to the battery core; low cost: low cost of PACK production line automation, including production equipment, production loss; easy split: battery pack Easy to maintain, repair, low cost, the battery core can be used well; to achieve the necessary heat transfer isolation, to avoid the thermal runaway spread too fast, you can also put this step into the pack design and then consider.

It is understood that at present, the efficiency of the modular group of the cylindrical core in the industry is about 87%, the system group efficiency is about 65%; the soft pack core mold group efficiency is about 85%, and the system group efficiency is about 60%. The square cell has a mode group efficiency of about 89% and a system group efficiency of about 70%. The individual energy density of the soft-packed battery has higher lifting space than the cylinder and the square, but the design requirements of the module are high, and the safety is not easy to control. This is a problem that needs to be solved by the structural design.

General module optimization approach, improving space utilization is also an important way to optimize modules. Power battery PACK companies can improve the module space and heat management system design, reduce the cell spacing, thereby improving the utilization of the space inside the battery box. There is also a solution that uses new materials. For example, the bus bar in the power battery system (the bus in the parallel circuit, usually made of copper) is replaced by copper into aluminum, and the module fixing parts are replaced by high-strength steel and aluminum by sheet metal materials, which can also reduce the weight of the power battery. .

Thermal design

The physical structure of the soft-packed battery determines that it is not easy to explode. Generally, only the pressure that the outer casing can withstand is high enough to be blown up, and the internal pressure of the soft-packed battery core is large, and the pressure and leakage will start from the edge of the aluminum plastic film. liquid. At the same time, the soft-packed battery core is also the best in heat dissipation.

The famous representative of the soft pack battery, the Nissan Leaf, its module structure is fully sealed, and does not consider heat dissipation, that is, does not dissipate heat. The capacity of Leaf's frequent feedback on the market is too fast, which is not unrelated to this thermal management. Obviously, with the pursuit of high-performance electric vehicles, the soft-packed batteries must also have an active thermal management structure.

The current mainstream cooling method has been transformed into liquid cooling and phase change material cooling. Phase change material cooling can be used in conjunction with liquid cooling, or alone in less harsh environments. There is also a process that is still widely used in China, which is filled with glue. Here, it is filled with a thermal conductive adhesive with a thermal conductivity much larger than that of air. The heat radiated by the telecommunications is transferred by the thermal adhesive to the module housing and further dissipated into the environment. In this way, the individual replacement of the cells is unlikely, but it also prevents the spread of thermal runaway to some extent.

Liquid cooling, in the picture of the module composition, the cold plate and the liquid cooling water pipe are the components of the liquid cooling system. The module is formed by laminating the cells, and there are liquid-cooled plates arranged at intervals between the cells, which ensure that each cell has a large surface contacting the liquid-cooled plate. Of course, it is not easy to make the liquid-cooled battery mature in the soft-packed battery. It must consider the fixing, sealing, insulation and the like of the liquid-cooled plate.

Electrical Design

Electrical design include low pressure and high pressure.

Low-voltage design generally requires consideration of several aspects of functionality. Through the signal acquisition harness, the battery voltage and temperature information are collected to the module slave control board or the so-called module controller mounted on the module; the module controller generally designs the equalization function (active equalization or passive equalization or both) Coexistence); A small number of relay on/off control functions can be designed on the slave control board or on the module controller; the module controller and the main control board are connected via CAN communication to transmit the module information.

The high-voltage design is mainly the series-parallel connection between the battery core and the battery core, and the connection between the design module and the module outside the module. The general module only considers the series connection. These high-voltage connections need to meet two requirements: First, the distribution of conductive members and contact resistance between the cells should be uniform, otherwise the cell voltage detection will be interfered; secondly, the resistance should be small enough to avoid the electrical energy on the transmission path waste.

Safety design

Safety design can be divided into three retrogressive requirements: good design to ensure no accidents; if not, accidents, it is best to give early warning, give people a reflection time; if the fault has occurred, the design goal will change To prevent accidents from spreading too quickly.

The first purpose is to achieve reasonable layout, good cooling system, reliable structural design; secondary targets require sensors to be distributed more widely to every possible point of failure, comprehensive detection of voltage and temperature, preferably monitoring each The internal resistance of a battery core; the lowest target, the fuse can be set through the battery core and the module, a firewall is set between the module and the module, and the design strength redundancy is used to cope with the possible structural collapse after the disaster occurs. This is the direction of the high performance soft package module.

Lightweight design

Lightweight design, the main purpose is to pursue the cruising range, eliminate all unnecessary burdens, and lightly go into battle. And if lightweight can be combined with cost reduction, it is even more joyful. There are many roads that are lighter, such as increasing the energy density of the battery core; in the detail design, the strength of the structural parts is ensured in the case of ensuring the strength (for example, selecting a thinner material and digging a larger hole in the plate); replacing with an aluminum material Sheet metal parts; shells are made from new materials with lower density.

Standardized design

Standardization is a long-term pursuit since the big industry, and standardization is the cornerstone of reducing costs and improving interchangeability. Specific to the power battery module, there is also a great purpose of using a ladder, having said that, the reality is that the cells are not standardized yet, so the standardization distance of the modules is even further.

The page contains the contents of the machine translation.