Pack is the process of assembling a lithium battery cell, which may be a single battery or a serial-parallel battery module. The spot weld process is important in the entire battery pack process.
Battery pack assembly involves 18650 cells, spot weld, PCB or BMS weld, bracket, and fish paper. However, the spot weld process is the core for battery pack assembly.
The spot weld process causes certain quality problems:
1. The nickel straps are overlapped in the process of welding (as shown in picture B). This cause the spot weld to be unstable and burnt. Finally，it will cause uneven resistance of each battery.
2. The spot weld over three times. Resistance weld is the need to weld the parts in connect pot and through the large voltage, high current, in contact with high heat for the parts of the contact with the local melting, and cooling after welding in overall. The battery pack causes an internal construction breakdown and leakage after multiple spot welds.
3. When the battery pack is in the spot weld, some battery pack manufacturers use single nickel straps (as shown in A-point and D-point). In addition, single nickel process causes battery pack different internal resistance.
The battery pack is charged and discharged throughout, causing the voltage gap because of internal resistance gap. After the charge and discharge cycles, it will reduce the life cycle as well.
Besides, when the battery pack works with the large discharge current, the nickel strap will be burnt.
4. There is no guide hole nickel strap (as shown in the B-point). The effect of no diversion hole is that when the spot weld machine flows rapidly to the cell, it will cause cells breakdown and even leakage
5. When the battery packs are in series and parallel, it is crucial to control each cell’s connection. This helps to avoid fake spot weld and reduces internal resistance. At the same time, it will reduce heating when the battery pack is operational. Also, when the battery pack works at a stable temperature, it will increase the life cycle.