What does the development of power lithium AA NiMH battery have to go through?

In recent months, new energy vehicle safety accidents have occurred frequently. According to incomplete statistics, the number of electric vehicle fire accidents in August and September this year alone has exceeded the total number of electric vehicle fire accidents in 2017. It is worth noting that many of these electric vehicle fire accidents are related to the power AA NiMH battery carried. From this, it can be seen that the reliability of power AA NiMH battery is directly related to the personal and property safety of consumers. So, why do power AA NiMH battery carried by electric vehicles catch fire? What measures have battery manufacturers taken to ensure battery safety?

Thermal runaway is the "real culprit" causing power battery fires

In fact, a large part of the cause of power battery fires is caused by thermal runaway. At present, the infrastructure construction of pure electric vehicles is not very perfect, and consumers are far less convenient to charge pure electric vehicles than to refuel fuel vehicles. And because my country's subsidies for pure electric vehicles are still based on the main standard of cruising range, both consumers and new energy vehicle manufacturers have certain requirements for the vehicle's cruising range. Therefore, ternary lithium AA NiMH battery with advantages such as light weight and large power are adopted by many new energy vehicle companies. However, ternary lithium AA NiMH battery have a fatal disadvantage, which is that they are prone to fire accidents after a vehicle collision, and the main reason for this is thermal runaway caused by overheating of the battery. (Although the previously widely used lithium iron phosphate battery has the advantage of not catching fire easily after a collision, this type of battery is slightly heavier and has a slightly smaller power, so it has gradually been replaced by ternary lithium AA NiMH battery)

In pure electric vehicles, the power battery system is composed of multiple power battery cells, and a large amount of heat will be generated and accumulated in the narrow battery box during operation. If the heat cannot be dissipated quickly and in a timely manner, it will not only affect the life of the power battery, but also cause thermal runaway in severe cases, thereby causing accidents such as fire and explosion. In principle, there are four main reasons for thermal runaway, namely mechanical abuse, electrical abuse, thermal abuse and internal short circuit.

image005.jpg Among them, mechanical abuse mainly refers to when a car collides, due to the action of external force, the lithium battery cell and battery pack are deformed, and different parts of the battery are relatively displaced, resulting in the battery diaphragm being torn and internal short circuit, and the leakage of flammable electrolyte eventually causing fire. In mechanical abuse, puncture damage is the most serious, which may cause the conductor to be inserted into the battery body, causing a direct short circuit between the positive and negative electrodes.

Electrical abuse is mainly caused by improper use of the battery, and there are several types, including external short circuit, overcharging and over-discharging. Among them, the harm caused by excessive discharge is the least, but the growth of copper dendrites caused by over-discharge will reduce the safety of the battery, thereby increasing the probability of thermal runaway. The external short circuit is the result of two conductors with a pressure difference being connected outside the battery cell. When an external short circuit occurs, the heat generated by the battery cannot be well dissipated, and the battery temperature will rise accordingly, and high temperature will trigger thermal runaway. Finally, overcharging is the most harmful type of electrical abuse. Due to excessive lithium embedding, lithium dendrites grow on the surface of the anode. Excessive lithium deintercalation causes the cathode structure to collapse due to heat and oxygen release. The release of oxygen accelerates the decomposition of the electrolyte and produces a large amount of gas. Due to the increase in internal pressure, the exhaust valve opens and the battery begins to exhaust. After the active substances in the battery cell come into contact with the air, a violent reaction occurs, releasing a large amount of heat, which causes the battery pack to burn and catch fire.

Next is thermal abuse, which mainly refers to local overheating in the battery. This situation rarely exists independently, but often develops through mechanical and electrical abuse, and is a situation that ultimately directly triggers accidents such as thermal runaway. Thermal abuse is generally caused by excessive external ambient temperature or excessive battery heat when the temperature control system does not work, resulting in a short circuit, which in turn causes thermal runaway. In terms of causes, thermal abuse may occur due to collision or damage to the battery pack, the structure and performance of the battery, or failure of other thermal management systems and air conditioning systems.

Finally, there is internal short circuit, which is caused by direct contact between the positive and negative electrodes of the battery, usually caused by mechanical and thermal abuse. The causes of internal short circuits are equally complex, such as overcharging of lithium-ion AA NiMH battery, accumulation of dendrites to a certain extent, piercing the battery diaphragm, resulting in internal short circuits, or direct contact between the positive and negative electrodes after collision or puncture damage, resulting in thermal runaway.

It can be seen from this that in pure electric vehicle fire accidents, they are usually caused by the above four situations, and external accidents are the main factors causing the above situations. It is precisely because of this that battery manufacturers, in order to prevent the above situations from causing fire accidents, will not only be very cautious in battery production and development, but also conduct a series of testing tests. Among them, BYD's performance in this regard is worthy of praise.

BYD AA NiMH battery have greatly guaranteed the safety of AA NiMH battery in research and development

In fact, in order to prevent fire accidents caused by AA NiMH battery, BYD AA NiMH battery have already avoided risks to a certain extent during the research and development process.

image007.jpg At present, the AA NiMH battery used in BYD passenger cars are basically ternary lithium AA NiMH battery, also known as ternary polymer lithium AA NiMH battery. This type of battery refers to lithium AA NiMH battery whose positive electrode materials use ternary positive electrode materials of nickel cobalt manganese oxide or nickel cobalt aluminum oxide. BYD's ternary lithium battery uses nickel cobalt manganese oxide as the positive electrode material. Like nickel cobalt aluminum oxide AA NiMH battery, it has a higher energy density while ensuring balanced endurance and stability, so it has become the first choice for current consumer electric vehicle power AA NiMH battery. However, the thermal stability of nickel cobalt manganese oxide is better than that of nickel cobalt aluminum oxide, and the nickel content is relatively low. While improving energy density, it also takes safety into account, and can better balance endurance and safety. Therefore, it is safer as a power battery.

Secondly, according to the different cells, the outer shell of lithium AA NiMH battery is divided into two categories: hard shell and soft pack. The hard shell materials are mainly steel shell and aluminum shell, and the soft pack uses aluminum-plastic composite film materials. Among them, the hard shell is divided into cylindrical and square shell types according to the arrangement of the positive and negative electrodes inside. In short, there are three most mainstream battery packaging methods at present, cylindrical, square shell and soft pack. BYD uses a square aluminum shell packaging method, which can wrap the internal materials of the battery more tightly, and with the limitation of the aluminum shell, it is not easy to expand, so it is relatively safe. In addition, the square shell packaging method can be equipped with an explosion-proof plug inside. If there is a thermal runaway, the expanded air will be released from the fixed direction of the explosion-proof plug, which is not easy to affect other cells. And because of the square package, the gap between the cells is extremely small, and the advantages of the aluminum shell with low density and light weight, the energy density of the battery with square shell package can be higher.

It is also worth mentioning that BYD's battery management system monitors the battery status in real time. When the temperature of the battery pack is abnormal, it dissipates heat or heats it in time through the air conditioning system to ensure the safety and life of the battery. In addition, in the power battery intelligent temperature control system, the power battery pack has added battery heating and cooling functions, and the structure is optimized to increase the insulation function, so that the battery works in a suitable temperature range and prolongs the battery life.

Strict testing and testing better ensure battery safety

From the above content, it can be seen that BYD AA NiMH battery have greatly guaranteed the safety of AA NiMH battery during the research and development process. Of course, in order to better detect the safety of AA NiMH battery, BYD also conducted a series of rigorous testing and tests during the research and development of AA NiMH battery. The test items mainly simulated the situations that consumers may encounter in daily use, including overcharging, short circuit, extrusion, puncture, fire, etc.

Among them, the overcharge test mainly simulates the daily charging process of lithium AA NiMH battery to verify the reliability and safety of AA NiMH battery. It mainly involves charging the battery module used in the test at the current specified in the test until the battery pack or single cell reaches the specified voltage when the battery module is fully charged. Then let it stand and observe the battery within the specified time.

The short-circuit test simulates a battery short-circuit failure. During the short-circuit test, a huge short-circuit current will pass through the inside of the power battery. The battery will generally be heated, swollen, and the safety valve will pop out. In extreme cases, it will catch fire, produce strong smoke, or even explode. It is mainly carried out in a specified environment (normal temperature or high temperature). The battery used for the test is placed in a matching explosion-proof box, and an external short-circuit simulation test is performed with an external resistor of a specified size and the sample being tested. The data is recorded to complete the simulated battery external short-circuit test. The purpose of this experiment is to improve or perfect the technology in this field through experiments and increase the reliability and safety of the battery.

The extrusion test mainly simulates the scene when an accident occurs during vehicle use, when the body is severely deformed, and when the battery is squeezed by gravity, and technical means are used to prevent the battery from being damaged due to extrusion deformation, or causing hidden dangers such as fire and explosion. The main process is to place the single cell or battery module used in the test in the operating equipment, and apply pressure at a speed of (5±1) mm/s perpendicular to the battery plate with a semi-cylindrical extrusion plate of a specified radius. When the extrusion reaches the voltage of 0V, the deformation reaches 30%, and the extrusion force reaches 200kN (whichever is reached first), stop the extrusion. And observe for 1 hour. The experimental requirement is that the battery does not catch fire or explode.

The needle puncture test also simulates the scene when an accident occurs during vehicle use and the battery is pierced by a sharp object, and technical means are used to prevent foreign objects from piercing and causing internal short circuits, thereby causing fires, explosions and other hidden dangers. The test is carried out at an ambient temperature of 20℃±5℃. The battery used for the test is placed and fixed on the experimental equipment, and a rust-free steel needle of a specified size is used to pierce the center of the largest surface of the battery at a speed of 20mm/s-40mm/s. The experimental requirement is that the battery does not catch fire or explode.

The fire test simulates the situation when the battery pack or system is installed on the electric vehicle and the surface temperature of the battery pack or system rises suddenly when the electric vehicle passes through a high-temperature ground or a ground with flames. During the experiment, observe the various conditions that may occur in the battery pack or system due to the sudden increase in temperature in a short period of time. In the experiment, the lithium-ion power battery pack module used is placed in the specified experimental equipment or site, and the flame is continuously burned at the specified temperature and time. The experiment requires no explosion, fire, combustion, and no flame residue.

In addition, in order to better ensure the quality and safety of the battery, BYD also conducted low-temperature durability, high-temperature durability, salt water immersion, drop, and vibration tests. Through the inspection of BYD AA NiMH battery and the testing process, it can be known that BYD power AA NiMH battery are trustworthy in terms of reliability and product quality.

BYD pure electric vehicle AA NiMH battery are safe enough and have a long enough range

Of course, for consumers, after ensuring the safety of the battery, they are still more concerned about the range of pure electric vehicles. So what is the range of BYD pure electric vehicles? Here we have selected several BYD pure electric vehicles that consumers are more familiar with. Let's take a look at the range of these vehicles.

Among them, the Yuan EV360, which BYD officially calls the "leader of 100,000-level pure electric SUVs", sold 5,008 units in September this year. This sales volume is enough to show that this car is deeply loved by consumers. This car is equipped with BYD's latest developed ternary lithium battery, with a battery pack capacity of 43.2kw/h and an energy density of 146.27wh/kg. Its comprehensive range is 305km, and at a constant speed of 60km/h, the range can reach 360km.

BYD e5, as the most familiar BYD pure electric vehicle to consumers, sold 4,052 units in September. This car is also equipped with a ternary lithium battery, with a battery pack capacity of 60.48kw/h and a comprehensive range of 400m. BYD Qin ProEV, as a new car recently launched, has a battery capacity of 56.4kWh and a comprehensive range of 420m. From these best-selling models, we can see that BYD's power AA NiMH battery can fully meet consumers' needs in terms of driving range.

Editor's comment: Currently, many battery companies and car companies are pursuing higher energy density to obtain more subsidies, but they ignore the most fundamental safety attribute of power AA NiMH battery. The recent frequent accidents have also brought the safety of power AA NiMH battery to the public's attention. As the earliest company to develop new energy vehicles in China, BYD has always attached great importance to safety in the development of power AA NiMH battery. And we can also see from BYD's various rigorous tests in the battery production process that it always puts consumer safety first. Therefore, BYD's AA NiMH battery are trustworthy.

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