Why does lithium battery module cell connection require laser welding?

Why does lithium battery module cell connection require laser welding?

Lithium power battery modules are assembled from multiple single cells connected in series and parallel. The single cells are connected and fastened, requiring small contact resistance between the connecting piece and the battery pole, resistance to vibration, and high reliability. Whether laser welding, resistance welding or bolt mechanical locking is used, the reliability and durability of the assembled battery system must be guaranteed during the actual driving of the electric vehicle. Among different power battery system design requirements, its volumetric energy density, mass specific energy density, and volumetric power density are all related to the connection structure and process between single cells in the power battery system.

Lithium power battery module is a power supply that connects several single cells in series and parallel through conductive connectors. It is fixed in the designed position through technology and structure, and collaboratively performs the function of charging, discharging and storing electric energy. It can be said that the basic functions of the module are connection, fixation and safety protection. Common module types can be divided into three forms: welding, screw connection, and mechanical crimping according to the connection method between the battery core and the conductive busbar. Studies have shown that the connection method between the battery cells and the module busbar not only affects the manufacturing efficiency and whether automation can be achieved, but also has an impact on the performance of the battery after it is installed in the vehicle that cannot be ignored.

However, it is not easy to ensure reliable electrical connections of lithium power battery modules, because the electrical connections of lithium power battery modules are closely related to the number of individual cells, connecting devices, connection methods, connection processes, connection materials, etc. that need to be connected. Related, the reliability of the connection not only depends on the material, structure and geometric dimensions of the connecting device itself, but also atmospheric pollution at the electrical connection point, such as dust, corrosive gases, humidity, and temperature, will directly affect the connection reliability. During the conduction process of the current in the lithium power battery module at the electrical connection parts, various random factors will affect the reliability of the electrical connection, such as current thermal effects, electrodynamics, external air pollution, temperature, humidity effects and electromagnetic interference. Causes unreliable electrical connections.

At present, there are two main methods of connecting polymer batteries: welding and non-welding (mechanical pressing contact type).

1) Welding. Welding includes laser welding or soldering.

Due to the large area of the power battery pack, the position of the ultrasonic welding head is not easily accessible, so ultrasonic welding is rarely used, and laser welding is an ideal welding method. The high-temperature process of soldering poses certain risks to the sealing of the tabs of the polymer cells. At the same time, the mass of the battery pack is increased due to the heavy proportion of tin. Regardless of the laser welding or soldering group process, it is not conducive to replace the monomer. Battery.

2) Mechanical crimping.

The connection structure and method of the polymer battery cell module that can be disassembled and replaced without welding is to use each battery cell as an independent unit, connect each unit in series and parallel, and ensure that each battery is detachable and replaceable. The main advantage of this process is that a single battery is removable and replaceable, which solves the problem of difficult replacement of single batteries after being assembled into a group, and improves the safety of the battery pack.

Single cells usually use a mechanical locking connection process. The advantage of this process is that it can be assembled and connected in a variety of ways, is easy to disassemble, and the assembly and connection is simple and flexible. The disadvantage is that due to its own structural limitations, compared to the flat-head type pole, The assembly process requires additional metal accessories, which increases the mass of the battery module.

The connection resistance of welding is lower than that of screw connection, which is a clear advantage of welding. At the same time, there is a lot of room for improvement in welding production efficiency. It can be said that in general, welding is better than screw jointing. But it can also be seen that screw connections are generally used in large batteries, where their stronger conductivity is highlighted and the disadvantage of low efficiency is weakened.

The advantage of mechanical crimping is that it is flexible in disassembly and assembly, and has a high success rate in subsequent maintenance and secondary recycling. The disadvantage is that it is difficult to significantly improve the assembly efficiency. If the mechanical connection structure design is not reasonable enough, there is a high possibility that the contact resistance will change in the long-term road vehicle operating environment.