8D Quality Report of
18650 2500Mah Battery High Resistance
On November 27th, we received one set of returned defective 18650-2500mAh battery packs (3S) from the customer. After confirmation, it was identified as a product batch from our company in May 2023.
Through internal resistance and voltage tests, it was confirmed that there is one battery in this set with abnormally high internal resistance, while the other two batteries are normal.
See the details in the following figure:
Team made up of colleagues from Quality, Production, R&D Department.
Root Cause Analysis
3.1 Analysis of Internal Resistance Defects
Upon dismantling the battery, it was observed that overwelding in the production of the negative tab resulted in an elevated internal resistance. As shown in the above image, the welding surface appears to be well done, but overwelding occurred, and during subsequent vibrations in the transportation process. This overwelded area gradually detached, leading to increased physical contact resistance.
4.1 It is recommended that after subjecting the entire batch of batteries to vibration, cells with internal resistances greater than 48mΩ should be downgraded.
4.2 It is suggested that the client performs performance/functional testing on their products, and any non-compliant items should undergo an evaluation process according to the corresponding rework procedure.
Potential Corrective Action
5.1 Due to the current use of 6-micron copper foil, there is a tendency to overweld during the welding of the negative tab. We have now made adjustments by changing the upper die head size from the original 3.5mm to 2.8mm. Additionally, the lower die has been switched from the original metal-coated die to a ceramic-coated die with improved thermal conductivity. (See the diagram for reference.)
5.2 The issue of transparency has also been improved and meets the required standards.
Implement Corrective Action
The vibration time was adjusted from the original 3 minutes to 5 minutes, and the vibration frequency was increased from 42Hz to 50Hz.
Take Preventive Action
Continued Follow-up and Improvement
Closure & Team Celebration
With the aforementioned measures, the issue of high internal resistance caused by overwelding in the production of the negative tab has been completely eliminated.
For the Original Report Document, please contact us.