The batteries used in mobile phones are all lithium batteries, which may explode under high temperature. In fact, they have happened and produced serious consequences. So most battery manufacturers will take protective measures. In addition to the structure design, IC over-current protection and MOS tube protection, secondary circuit protection elements will be used to strengthen the protection measures.
There are three kinds of circuit protection components commonly used in mobile phone battery: current fuse, temperature fuse and positive temperature coefficient thermistor. Different components reflect different protection requirements and different design concepts.
Current fuse is mainly used for short circuit protection
The protection principle is that once the battery has a short circuit, the large over-current quickly breaks the heat balance of the normal work, instantly makes the melt temperature rise to the melting point of the material, the circuit will be disconnected, so as to achieve the function of safety protection. The advantage of using current fuse protection is that it can cut off the circuit quickly and thoroughly, and the precision of fusing characteristic is high. No matter when charging, using or standby, as long as any part of the short circuit will act quickly. However, it is only sensitive to current and has no response to external temperature.
Temperature fuse is mainly used for overheating protection
The principle of protection is that the melt made of low melting alloy will melt when it meets the allowable temperature, so as to cut off the circuit and achieve the function of safety protection. The advantage of using temperature fuse protection is that the melt is sensitive to the internal and external temperature of the component, especially when the mobile phone is placed in a high temperature environment or too high temperature for other reasons. However, the response to the over temperature caused by over-current is relatively slow, the accuracy is low, and the reliability is not as high as that of current fuse.
Positive temperature coefficient thermistor is mainly used for overcharge protection
The protection principle is to use the positive temperature coefficient and Curie point mutation characteristics of PTC material resistance, that is, the temperature rise caused by any reason (including over-current and environment) will cause the material resistance to rise. Once it rises to Curie point, the resistance will become large enough to turn off the charging current and achieve the function of safety protection. The advantage of using thermistor protection is that it can respond to current and temperature, and can timely limit the current when the battery is overcharged. The disadvantage is that the response time is slow, there is a small leakage current when it is turned off, and the reliability is not high enough.
The temperature coefficient thermistor is also called self recovery fuse, but in the protection of mobile phone battery, self recovery function is not necessary, especially the over-current caused by circuit fault. If the fault is not eliminated and PTC self recovery, it has no significance for practical application.
