Since 2015, electric car burnt events have been repeatedly exposed by the media. How to solve this problem, the first national lithium-ion battery safety technology seminar hosted by Tsinghua University was held in Beijing on May 6-7, 2017. Professor Ouyang Minggao believes that solving the problem of thermal battery thermal runaway needs to be solved. Three axe. These instructions must solve the safety of the power battery on electric vehicles. First, the industry must attach great importance to it. Second, it is necessary to develop a system with high technical indicators (assembly parts). Actively configured to the car. The author will share with you the power battery safety intelligent system and case analysis and suggestions for your reference.
The power battery is an energy carrier, indicating that the cause of its safety hazard is objective. According to Professor Ouyang Minggao's summary, it can be roughly divided into three categories:
1) External high temperature will cause the explosion-proof membrane of the battery to rupture, and the electrolyte will be ejected, which will cause burning and fire. In an external high-temperature environment, the electrolyte decomposition product reacts with the positive electrode and the negative electrode, and the cell diaphragm melts and decomposes to cause an internal short circuit, and the discharge of electric energy also generates a large amount of heat.
Figure 1 Schematic diagram of test data
The test data shows that when the cell temperature reaches 135 ° C, the diaphragm begins to melt and the voltage drops; the battery voltage drops rapidly at 150 ° C, and the diaphragm collapses completely when the temperature reaches 245 ° C. The conclusion is that the battery cell operating temperature is between 40 ° C ~ 50 ° C is ideal. The conclusion is that if the temperature range is above or below this, it is not conducive to the normal use of the battery.
2) Impurities, metal particles, charge and discharge expansion and contraction, and lithium deposition inside the power battery may cause internal short circuit of the power battery. The mechanism of the process of internal short circuit caused by impurities inside the power battery is still in the research stage.
The conclusion is that at present, it is necessary to eliminate the incentives such as impurities inside the power battery, and theoretically, no feasible solution has been found.
3) Damage to automobile machinery such as strong impact, rollover, and extrusion are also objective facts without circumvention. Damage to the machine can cause mechanical damage to the battery, and in a short period of time, a short circuit in the power battery can occur. The conclusion is that the car is a moving object and the mechanical damage of the power battery is also a high probability event.
Second, the human factor of the safety accident of the power battery1) External short circuit due to improper charging operation. The battery discharge current is very large, which will cause the battery core to heat up. The high temperature will cause the diaphragm inside the battery core to shrink or completely deteriorate, causing an internal short circuit and thus exploding. The conclusion is that cultivating and strengthening management is a very necessary basic management task.
2) Overcharging of the battery causes the lithium of the positive electrode to be excessively released, which causes a change in the structure of the positive electrode. However, too much lithium is easily released into the negative electrode, and it is easy to cause lithium deposition on the surface of the negative electrode. Moreover, when the voltage reaches 4.5 V or higher, the electrolyte will decompose to produce a large amount of gas, thereby causing the battery to swell, and even if it is serious, it may even take off. The smoke caught fire. The conclusion is that it is necessary to tell the operator in time through measures such as sound and light, and the technical measures are immediately in place. It is necessary to prevent this behavior from happening.
3) Over-discharge will cause damage to the molecular structure of the battery cathode material, resulting in no charge. The conclusion is that it is necessary to tell the operator through measures such as sound and light, and at the same time, technical measures must be taken to prevent this from happening.
4) The IP protection level is not up to standard, resulting in excessive moisture content inside the battery. The water can react with the electrolyte in the battery to produce a gas. When charging, it can react with the generated lithium to form lithium oxide, which causes the capacity of the battery to be lost. It is easy to overcharge the battery to generate gas and decompose the water. The voltage is low, and it is easy to decompose and generate gas when charging. When the gas generated in this series will increase the internal pressure of the cell, the cell will explode when the cell's outer casing cannot withstand it.
Third, power battery safety and security measures1) The BMS management system should have the function of automatically circumventing man-made improper operations.
2) Process control measures in the production of power battery cells are in place. The main thing is to ensure that the quality of the monomer is high and the consistency is high. The IP rating of the battery pack must meet the technical requirements of the electric vehicle;
3) The BMS management system should be able to control the ambient temperature of the electric system, and it should be kept properly with automatic adjustment function;
4) Under low temperature environment, the BMS management system should be able to automatically warm the power system, and it should be able to eliminate these automatically when the burning (or explosion time) is about to occur;
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