The cell of the 18650 lithium-ion battery has two layers of protection in the internal hardware design, namely the CID (Current Interrupt Device) pressure relief safety valve and the PTG (Positive Temperature Coefficient) thermal resistor. Among them, the safety valve is a standard feature of every 18650 battery, and it is also the most important explosion-proof barrier.


The working principle of ClD and PTC: When the internal temperature of the battery cell is abnormal, a large amount of gas appears due to overcharging, short circuit, etc., and the air pressure rises to 1.0-1.2Mpa, the unified aluminum sheet on the safety net will bounce upward and contact the bottom The aluminum sheets need to come out of contact to open the circuit immediately. If the pressure continues to rise, the safety valve will break and open to remove the internal gas to release the internal pressure to prevent an explosion caused by excessive pressure. When the internal air pressure of Prismatic square soft-coated battery cells increases, the most common phenomenon is "pregnancy" bulging. Secondly, the PTC thermal resistor. When the battery input and output current is too high, the PTG will heat up. When the temperature reaches the preset value, the PTC resistor will suddenly increase, cutting off the external current input to the battery core or preventing the current output in the battery core. , so that the battery core stops working. In addition to the above two, batteries also have a third line of defense that is unknown - the separator. When the battery heats up to above 160 degrees Celsius, the micropores in the separator will close, causing the positive and negative electrodes to be physically isolated, and the battery will naturally no longer output current.


Since the 18650 battery cell itself is so safe, why are mobile power supply fire accidents still not uncommon? The secret lies in the power bank itself. Plastics are often used in components such as external chargers of mobile power supplies. Plastic itself is flammable and can spontaneously ignite when exposed to high temperatures. The USB output of a qualified mobile power supply has short-circuit protection, so when the output is short-circuited, no large current will pass through. But there are also connections from the 18650 to the circuit board inside the mobile power supply such as chips or wires. If this is unfortunately short-circuited, then the output short-circuit protection is completely powerless. The PTC of the 18650 battery cell itself can only cut the current of the new battery before the battery itself overheats. Since the heat capacity of the battery is considerable, the temperature does not rise that fast. Perhaps when the wires and cells are burning red, the battery has not yet heated up. Cut new output. In this way, the red-hot chip comes into contact with the filled plastic, and a fire is inevitable.


Qualified mobile power supplies often provide multiple insulation for the conductor from 18650 to the circuit board, and the insulator used is high-temperature polyimide tape (Mylar sheet) that can withstand 300 degrees and flame-retardant green paper. At the same time, this section of conductor has an appropriate fixed design to prevent the conductor from moving or deforming under mechanical impact. This ensures that short circuits will not occur to the greatest extent.
Low temperature lithium iron phosphate battery 3.2V 20A -20℃ charging, -40℃ 3C discharge capacity ≥70%

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