A simple analysis of three commonly used components for 18650 battery 3.7v 2000mah circuit protection

　　The batteries used in mobile phones are all lithium batteries, which may explode at high temperatures. In fact, this has happened and has had serious consequences. Therefore, most battery manufacturers will adopt protective measures. In addition to structural design, IC overcurrent protection and MOS tube protection, secondary protection components will also be used to strengthen protection measures.

　　There are three commonly used circuit protection components on mobile phone batteries: current fuses, thermal fuses and positive temperature coefficient thermistors. The use of different components reflects different protection requirements and different design concepts.

　　Current fuses are mainly used for short circuit protection:

　　The protection principle is that once the battery is short-circuited, the large overcurrent quickly breaks the thermal balance during normal operation, instantly raising the melt temperature to the melting point of the material, and the circuit will be disconnected, thereby achieving the safety protection function. The advantages of using a current fuse for protection include fast and complete circuit cutting and high precision of the fusing characteristics. Whether it is charging, in use or in standby, any short circuit in any part will act quickly. But it is only sensitive to current and does not respond to external temperature.

　　Thermal fuse is mainly used for overheating protection:

　　The protection principle is that the melt composed of low-melting electrical alloy material will melt once it encounters a temperature exceeding the allowable temperature, thereby cutting off the circuit to achieve safety protection. The advantage of using a thermal fuse for protection is that the melt is sensitive to both the internal and external temperatures of the component. Especially when the mobile phone is placed in a high-temperature environment or is overheated due to other reasons, it will act. However, the response to overtemperature caused by overcurrent is relatively slow. Slow, less accurate, and less reliable than current fuses.

　　Positive temperature coefficient thermistor is mainly used for overcharge protection:.

　　The protection principle is to utilize the positive temperature coefficient characteristics of pTC material resistance and the mutation characteristics of Curie point. That is to say, any temperature rise caused by any reason (including overcurrent and environment) will cause the material resistance to rise. Once it rises to the Curie point The resistance will become large enough to shut off the charging current and achieve safety protection function. The advantage of using thermistor protection is that it responds to both current and temperature, and can limit the current in a timely manner when the battery is overcharged. The disadvantage is that the response time is slow, there is still a small leakage current during shutdown, and the reliability is not high enough.

　　The positive temperature coefficient thermistor is also called a self-restoring fuse, but the self-restoring function is not necessary in the protection of mobile phone batteries, especially for overcurrent caused by circuit faults. If the fault is not eliminated and the pTC self-restorates, it will be detrimental to practical applications. It doesn't make sense.

　　There are also some low-end mobile phone batteries that, in order to save costs, do not use any protective components, or use zero-ohm resistors as protective components to deceive supporting factories or customers. This is irresponsible and poses considerable safety risks.

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