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Factors affecting the safety of large-capacity mobile power supplies

by:dcfpower     2021-03-10
With the continuous development of mobile devices, mobile power has gradually become one of the indispensable devices in people’s lives. However, with the gradual expansion of its application range and the increasing capacity of a single battery, the safety of mobile power is also a problem. People are paying more and more attention. There are three main aspects that affect the safety of the mobile power supply: 1. The rationality of the PCB board design; 2. The selection and usage specifications of the internal batteries of the mobile power supply; 3. The correctness of human operation. Among them, the design of the PCB board should be reasonably designed according to the selected battery core and charging and discharging requirements, and at the same time, the voltage and current control and protection circuit settings should be realized. If the protection settings of the PCB board are realized, there will be no safety accidents under normal circumstances. Therefore, this article mainly focuses on the safety issues of mobile power batteries. There are two common mobile power batteries on the market, one is a lithium battery pack and the other is a polymer lithium battery pack. The following is a brief overview of the safety of the two batteries:

One, mobile power using lithium battery pack cells (1) internal factors affecting the safety of lithium-ion batteries 1. Materials. (1) The thermal stability of the cathode material and its reaction activity with the electrolyte. Most of the positive electrode materials have good thermal stability when in the discharged state, and exhibit thermal instability when in the charged state. When not in contact with the electrolyte, the main cause of heat generation of the charged positive electrode material is due to the transformation of the crystal form. After contact with the organic electrolyte, the charged positive electrode reacts with the electrolyte to oxidize the electrolyte and release a large amount of heat. (2) The safety of the negative electrode material. The negative electrode material should have good chemical stability, be able to form an SEI film with the electrolyte and not react with the electrolyte, otherwise the large amount of heat generated by the reaction may cause the battery to explode. (3) The composition of the electrolyte. The electrolyte is responsible for the charge transfer between the positive and negative electrodes inside the battery. It requires high conductivity and the chemical properties of the electrolyte must be stable to minimize the electrochemical reaction rate between the electrolyte and the active material interface during storage. The self-discharge capacity loss is small. At present, the electrolyte of lithium batteries contains a certain amount of acidic substance HF, and the positive electrode material may be dissolved and corroded. When the positive electrode material is dissolved, the internal resistance of the battery will increase, and the poor thermal stability may easily cause safety hazards. (4) The choice of diaphragm. When the temperature in the cell is close to the melting point of the polymer diaphragm, the polymer matrix ion-conducting polymer film melts, and the holes will be blocked and become a non-porous insulating layer. The internal resistance of the battery rises rapidly, and the current through the battery is limited. ; When the temperature continues to rise to exceed the melting point of the polymer film, the polymer film further melts, causing the positive and negative electrodes to communicate directly, causing an internal short circuit, and finally causing the battery to explode. Therefore, the melting point, thickness, porosity and other issues of the diaphragm should be considered in the selection of the diaphragm. 2. The rationality of the battery structure design. Lithium-ion batteries usually adopt safety valves, thermistors, and design protection circuits to protect them. The protection circuit is a PCB circuit composed of a variety of electronic components. It controls the charging and discharging process of the battery to prevent the battery from overcharging and discharging. Each of the components has the possibility of failure. Once the protection circuit fails, the probability of an accident in the battery will be greatly increase. 3. Production process. There are many noteworthy details in the manufacturing process. These details can easily cause safety hazards, such as: pole piece burr, pole powder falling off, etc., which will pierce the diaphragm and cause internal short circuit; battery pole ear is too long to contact the pole piece or the shell Causes a short circuit; the tabs compress the winding core, resulting in a short circuit between the positive and negative electrodes, causing the battery core to become hot, and in severe cases, it will cause an explosion; the inappropriate amount of positive and negative materials during the production process will cause lithium to be released, and piercing the diaphragm will easily cause internal short circuits. . At the same time, the use of secondary batteries should be avoided to avoid safety accidents such as short circuits. (2) External factors that affect the safety of lithium-ion batteries. One is overcharge and overdischarge. Overcharge or overdischarge can easily cause the positive and negative electrode materials or electrolyte in the lithium-ion battery to decompose and release gas and heat, which will cause the battery to swell, and the internal pressure will rise sharply. In severe cases, it will even cause smoke, fire and explosion; the second is external Short circuit. External short circuit The positive and negative electrodes are short-circuited due to improper operation or failure of the protection circuit board. When the short circuit occurs, the battery discharge current is very large, which causes the battery core to heat up. ; The third is overheating. Excessive external temperature may cause deformation of the metal shell, melting of the internal diaphragm of the battery, decomposition of the positive and negative materials or electrolyte, etc., all of which can easily cause the battery to short circuit. 2. Mobile power uses polymer battery cells. Polymer batteries are safer than lithium batteries. The outer packaging of the polymer battery is generally made of aluminum plastic film. In the event of a short circuit or other abnormalities, the inside will swell, breaking through the hot melt and venting, generally there will be no explosion accident, but when the battery is severely short-circuited Fire and smoke occurred, so the battery itself did not explode. The explosion was caused by the explosion of other parts due to the burning of the battery.
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