The battery pack swells after half a year, and it's mostly due to these three reasons.

Recently, I've been repairing several battery packs with bulging issues, and the reasons are highly consistent. I'm writing this to help everyone avoid pitfalls. Firstly, the internal resistance was not measured during assembly, and the weaker cell was subjected to long-term over-discharge; secondly, the protection board's current was selected too small, causing the protection board to overheat under high current conditions; thirdly, charging them in the trunk during summer directly exposed them to an environmental temperature exceeding 45°C.