Recently, electric cars have been on fire again. The battery fire has added another anxiety to consumers who have just dropped their mileage. What kind of power batteries are there, what are their advantages and disadvantages, and why do they catch fire? Today we take everyone from the battery to the battery pack to analyze one by one.
First, the type of power battery
First of all, the batteries used in our common new energy electric vehicles are relatively harsh in use and have high requirements on some parameters such as capacity, volume and weight. Among them, the battery's main performance indicators include: voltage, capacity, internal resistance, energy, Energy density, power, and service life. Among the many recyclable batteries, lithium battery performance parameters are most suitable for the use of pure electric vehicles.
Lithium-ion batteries are a new generation of batteries, and they are also batteries that use more electrical appliances with higher capacity requirements. In new energy electric vehicles, lithium iron phosphate batteries and ternary lithium batteries are mainly used. The so-called lithium iron phosphate batteries refer to lithium ion batteries using lithium iron phosphate as the cathode material. In the early days, due to their low cost (excluding rare precious metals, phosphorus , Rich iron resources, low price), good thermal stability, long cycle life and other advantages, lithium iron phosphate batteries are used as power batteries.
However, as users' requirements for cruising range increase, in the field of passenger cars, relatively low energy density lithium iron phosphate batteries are gradually being replaced by ternary lithium batteries. In commercial vehicles and buses, lithium iron phosphate batteries will have some applications, mainly for vehicles with relatively fixed driving paths or short-haul transportation, because low cost and high safety are more important for commercial purposes.
The full name of the ternary lithium battery is a lithium ion secondary battery using a ternary polymer such as nickel nickel cobalt manganate (NCM) or lithium nickel cobalt aluminate (NCA). The ternary lithium battery is divided into multiple systems according to the mixing ratio of the positive electrode materials, such as NCM811, NCM622, and NCM333. The number represents the mixing ratio of the three elements. For example, the nickel, cobalt, and manganese in NCM811 uses a mixing ratio of 8: 1: 1. , Is currently the highest energy density of nickel-cobalt-manganese ternary lithium battery. However, there are pros and cons to adjusting the ratio. As nickel (Ni) increases, the capacity increases, but the stability also decreases, and battery safety is affected. For new energy vehicles, the improvement of cruising range is a hard indicator, and safety is more important.
Second, the characteristics of different shape batteries
New energy electric vehicles require high specific energy, large specific power, high charge and discharge efficiency, good relative stability, low use cost, and good safety. With so many requirements, the requirements for batteries are very strict. The battery cells in the car do not exist independently. They need to be arranged into a battery pack. Combined with the body structure, heat dissipation system and electronic control system, a closed battery pack is formed to power the vehicle.
The battery pack of most pure electric vehicles is installed at the chassis. The main reason is that the chassis is relatively flat and has a larger area. The battery pack is arranged on the side, but it also brings safety risks. It is inevitable that the chassis will be bumped during driving. The difference between fuel vehicles is that the position of the battery pack may be lower than the fuel tank, it is easier to be bumped, and it is easy to cause extreme conditions such as short circuits in the battery pack.
According to demand, battery manufacturers pack batteries into a variety of shapes to meet the needs of different models. At present, cylindrical, square, and soft-pack batteries are commonly used. Car companies will design battery packs according to different models, and select the appropriate battery cells to be placed on the battery. Inside the group.
The advantages and disadvantages of various shapes of cells compared with the advantages and disadvantages of the manufacturing process. Cylindrical round winding type has good consistency, mature technology, low cost, flexible composition. The heat dissipation system is complex, and the energy density is low. The material of the shell is mostly steel, resulting in energy density. The low-square-square-wound type has good heat dissipation performance, flexible composition, and high rigidity of the shell to enhance safety. The relatively fixed size causes high costs. The steel shell is too heavy. Currently, it is partially replaced with an aluminum shell. High, flexible size change, poor strength, easy to leak, easy to leak, complex heat dissipation system after grouping, poor consistency. Aluminum plastic film is the outer packaging of the battery, which is vulnerable, suitable for solid-state batteries.
Our common model of cylindrical battery is 18650. This set of numbers represents the diameter of the cylinder is 18mm, the length is 65mm, and 0 is a round cylindrical battery. 18650 battery has the characteristics of high power output and high energy density, which meets high battery life and fast charge and discharge.
Due to the high number of battery cells required for battery packs, it is necessary for batteries to be suitable for large-scale standardized production to meet the supply. It is precisely cylindrical batteries that meet this requirement. However, cylindrical batteries also have disadvantages. Too large a number of cells will complicate the management system of the battery pack, difficult to design the heat dissipation method, and affect the energy density of the battery pack. Of course, the consistency of the battery cells is also very important. A large number of cells means a higher failure rate. A battery cell has a problem, which can cause the battery capacity to decrease, and it can cause overheating and fire. The higher the energy density, the more heat generated by the discharge, which means that it is prone to thermal runaway.
At present, there are many manufacturers of home appliance cores in China that provide batteries for pure electric vehicles. In a battery pack, a battery module is composed of multiple battery cells, but usually an OEM only has the ability to package battery modules, and cannot produce battery cells.
From this point of view, whether a model can be successfully launched and mass-produced will be greatly affected by the quality and capacity of battery cells provided by battery cell manufacturers, and will ultimately directly affect the battery pack output of new car manufacturers and new cars. Production delivery.
Tesla is a brand that currently uses cylindrical batteries on a large scale, but Tesla uses batteries from Panasonic's NCA (nickel cobalt aluminum) ternary lithium batteries.
Take the Tesla ModelS as an example. The battery pack contains more than 7000 18650 cells. A large number of battery cells bring greater pressure to the battery pack's heat dissipation and management system. On the Tesla Model 3 that was launched this year, 21700 cells have been used instead, and the number of battery cells has also been greatly reduced. It can be understood that the management difficulty and failure rate of battery cells are reduced, and the safety of the entire battery pack is improved.
Square case battery:
The biggest advantage of the square case battery is that it is convenient to arrange and design the overall shape of the battery pack because of its regular shape. The heat dissipation of the square case battery is easier to design, and the reliability is good. There is a pressure relief valve. Overheating in a short time can release the pressure as soon as possible to ensure the safety of the battery pack. However, there are many shell models of square-shell batteries, and their sizes are relatively fixed, resulting in a slightly higher cost.
At present, many domestic brands will use square-case batteries as the battery cells. The battery cells have certain safety measures to ensure safety, improve the safety of the entire battery pack, and make the shape more easily.
The battery pack of BYD Qin ProEV500 uses independently developed nickel-cobalt-manganese ternary lithium batteries, and is packaged in the form of a square case. The battery case made of magnesium aluminum alloy is heavier than the stainless steel case used for cylindrical batteries. Lighter and helps increase energy density.
In the end, Qin ProEV500's battery pack has an energy density of 160.9Wh / kg, a capacity of 56.4kWh, and can provide 420km endurance under NEDC conditions.
Soft case battery:
Because the soft-pack battery does not have a heavy casing, the relative weight is lighter, the shape of the battery can also be flexibly changed, the adaptability is higher, and the energy density is higher. However, there are advantages and disadvantages to everything. Without an outer shell, it only relies on aluminum plastic film as the outer packaging, which makes the mechanical strength of the battery short, and the sealing process is more difficult, resulting in a poor consistency of the battery.
The poor consistency of the soft pack battery will cause the stability of the entire battery pack to decrease. The biggest problem is temperature control. If the temperature of individual cells is too high, it may cause a fire, or the battery management system will ensure that the temperature of a certain cell is within a safe range, which will cause the performance of the entire battery to decline. Therefore, if you want to use a soft pack battery with high energy density, you need to cooperate with an efficient heat dissipation system and battery management system.
The metal sheet with water channel in the picture is a special designed cooling sheet for soft pack batteries. It can introduce cooling liquid into the water channel to dissipate heat, ensuring that the battery is always at an appropriate working temperature. Finally, a frame is used to combine the battery core, cooling fins, and foam that plays a role in heat preservation and pressure, forming an independent stacking unit.
Combine several battery stacking units, install related connection harnesses, voltage and current temperature acquisition modules, relay assemblies and other related parts to ensure that the working status of each battery cell can be monitored in real time and controlled in a targeted manner, and finally formed A car battery module.
The soft pack cells and battery packs introduced above are the first pure electric product VELITE6 from SAIC-GM-Buick. At present, most pure electric vehicles using soft pack batteries in the industry are from joint venture brands, which has stimulated the domestic soft pack battery market. Although the soft pack battery has a high energy density, the soft pack battery has high requirements on the heat dissipation performance of the battery management system and the battery pack, and it will take some time for the widespread use.
Third, about the vehicle fire
At present, we can see several electric vehicle fire accidents. One is that the vehicle is in an abnormal state and spontaneously ignites in a static state; the other is that the vehicle battery pack itself is damaged, such as a collision or puncture. In the case of spontaneous combustion and fire.
After the battery pack catches fire, it belongs to an anaerobic combustion state, and the speed from visible smoke to open flame formation is very fast. Due to anaerobic ignition, conventional dry powder fire extinguishers and carbon dioxide fire extinguishers cannot play a role, and fire hydrants can only be used as soon as possible to reduce the fire temperature. However, the battery pack is a closed design, and the installation location is mostly at the bottom of the vehicle. It is difficult for the spray to directly affect the ignition point, which is why the fire suppression efficiency is low.
In summary, in order to avoid harming innocent people due to the hidden dangers of individual vehicles, should future pure electric vehicles be placed in special parking spaces when parked? Should the frequency of vehicle inspections be increased to monitor the condition of the battery pack? These are all issues we should consider in the use of pure electric vehicles in the future.
Fourth, some suggestions for using electric vehicles
1. In the daily use process, if undercarriage or any collision caused by the chassis, go to the 4S shop as much as possible to have the vehicle inspected by professionals. Although it is complicated but careful to drive the ship for ten thousand years, the car with the hidden danger may be injured. And itself and even the surroundings, such as the Tesla basement fire.
2. At the present moment, the summer will immediately begin, and the temperature will gradually rise. Although new energy electric vehicles do not have an internal combustion engine, high temperatures put the battery pack on the chassis to the test. Many cities in the north will have flying skies. Many new energy vehicles use water-cooled design to cool the three-electric system. If a large number of flying skis block the water tank on the front of the car, it may cause poor heat dissipation and affect the temperature control of the battery pack.
V. On the claims after spontaneous combustion
In the business scope of vehicles and auto insurance, spontaneous combustion is a special case. First of all, in the case of automobile damage insurance in the basic types of insurance, the scope of claims includes collision, overturning, falling, fire, etc., but does not include spontaneous combustion.
Because the cause of spontaneous combustion is more complicated, it may be due to the quality of the vehicle itself, or it may have been modified by the owner in the later use process, and there are hidden dangers such as car circuits that cause fire. However, if the vehicle spontaneously ignites, losses can be avoided through corresponding additional risks, such as spontaneous combustion. The scope of liability is in the absence of an external fire source, due to the vehicle's own electrical appliances, circuits and other related systems, causing the vehicle to ignite spontaneously, including the loss of the vehicle caused by the fire of the cargo itself, which is also covered by the claim.
The identification of the responsibility for a car spontaneous combustion accident must first determine that it is spontaneous combustion. In other words, the fire caused by the defect of the vehicle rather than the fault of the user. At the same time, the fire department, the traffic police department, or a qualified third-party appraisal agency identified the cause of the fire, and then determined that the fire accident was the responsibility of the car manufacturer or owner.