What is PTC’s function in cylinder lithium battery? Such as in 18650 and 21700 etc.

The main function of installing PTC components on cylindrical lithium batteries is to protect the battery from damage or safety hazards in abnormal situations. Specific functions include:
Overcurrent protection
PTC components monitor battery current, and when abnormal overcurrent is detected, their resistance rapidly increases to limit the current and prevent battery damage due to overheating. This characteristic is based on the positive temperature coefficient of PTC material, which can respond and cut off the circuit in a short time.
Short circuit protection
When a short circuit occurs inside or outside the battery, PTC components block the current by changing the resistance value, avoiding high temperature induced thermal runaway. For example, polymer PTC components can restore their conductive state after a short circuit is eliminated, achieving repeatable protection. ‌
Temperature control
PTC components are located near areas of the battery that are prone to heating, and can detect temperature anomalies in a timely manner through changes in resistance. For example, the PTC starting current of small capacity cylindrical batteries is smaller, allowing for more precise temperature regulation. ‌
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Security Enhancement
In electric vehicle battery packs, PTC works in conjunction with battery management systems to avoid the risk of fire or explosion caused by overcharging, short circuits, and other issues