Reliability analysis of three-electric system for long-distance patrol car

After the COVID-19 outbreak, I was trapped in a small room for nearly three weeks. The scenery on the road that I usually don't pay much attention to now makes me feel a long-lost fresh breath. Fortunately, as a car owner, I can drive to work, stay away from public transportation and avoid cross-infection. A car has brought me all-round protection to return to work.

It can be said that during the anti-epidemic period, a car can bring double safety protection to itself and family besides a mask.

Speaking of safety protection, as a person in the automobile industry, compared with traditional fuel vehicles, the safety performance of new energy electric vehicles is also my key concern. Under the electrification frenzy, in 2020, joint venture brands, domestic independent brands and foreign brands will also launch their flagship new energy vehicles according to the strategic plan.

Before electric vehicles are put on the market, they will go through complicated and strict safety testing and certification, just like traditional fuel vehicles. Volkswagen, GM, Honda and Toyota are all deeply involved in the direction of electric vehicles, among which the safety of electric vehicles is the most important.

Stakeholders, as insiders, have the most contact with the project comprehensive department. Speaking of it, GM's promotion of pure electric vehicles in China has only begun to develop in the past decade. In fact, as early as a century ago, GM has begun to explore the future of electrification, which can be said to be one of the founders of electric vehicles.

It is reported that at the end of February 2020, GM will soon launch the "Chevrolet Patrol" model. Chevrolet Toure, positioned as a pure electric intercity coupe, combines the superior resources of SAIC-GM's parent company in new energy and car networking technology. Different from the traditional drive technology, the power is composed of intelligent three electric systems, namely, high-performance electric drive system, 3x3 electric control system and ternary lithium battery system. With the effective cooperation of the three systems, drivers and passengers experience an unprecedented sense of driving, bid farewell to the burden of energy consumption and become environmental pioneers.

So, do ordinary engineers use these three electric black technologies to ensure the safety of every passenger?

As an insider, I got some internal information through various channels. Here, I would like to focus on the following: Chevrolet's three electric systems are developed in strict accordance with the system safety requirements higher than functional safety, and the battery pack is newly equipped with an intelligent water circulation temperature management system, which meets the extreme safety detection far beyond the national standard requirements and is equipped with a triple physical protection structure design to fully meet the safety of the whole vehicle.

Summary: Industry-leading battery management system and highly complete and reliable power battery pack.

1. What is Sandian? Sandian is the core system of electric vehicle.

Bottom line: power battery, electric drive system and electric control system.

It sounds easy to understand. First, I will give you a simple knowledge of science popularization.

1. 1 power electricity?

At present, automobile power battery is basically composed of the following five systems: power battery module, structural system, electrical system, thermal management system and BMS battery management system (electronic control).

Compared with the human body, the module is the "heart" of the power battery, which is responsible for storing and releasing energy and providing power for the car. A lithium battery module consists of several to hundreds of battery cells connected in parallel and in series. In addition to the mechanism design, the battery management system and thermal management system can form a relatively complete lithium battery pack system.

At present, mainstream power battery modules are as follows:

Lithium cobaltate 18650 battery: The production process is mature, and the energy density of the battery is high, which is nearly twice that of the lithium iron phosphate battery of the same grade, but the stability at high temperature is worse than other batteries, and the battery is easy to catch fire. Representative model? Tesla? model

Ferric phosphate battery: It is the safest and most stable lithium battery for vehicles. In extreme cases, passengers in the car can be guaranteed to be protected from battery explosion or secondary fire to the greatest extent. Representative models BYD Qin and Tang.

Ternary lithium battery: The safety and energy density are between lithium iron phosphate battery and lithium cobalt oxide battery 18650, which are relatively balanced in all aspects, and the battery is more stable at low temperature. Representative models BYD mixed Song Yuan, Beiqi EV200, EV260, Tesla models? three

Ni-MH battery: It has the advantages of long service life, multiple charge and discharge times, maintenance-free and relatively good performance index for a long time, but it is large in size and low in energy density. Representative models Toyota Prius, Ford Escape, Chevrolet Malibu and Honda Civic? Half-breed.

structural system

It is mainly composed of battery cover, tray, various metal brackets, end plates, bolts, etc. It can be regarded as the "skeleton" of the battery pack, which plays the roles of supporting, resisting mechanical shock and vibration, and protecting the environment (waterproof and dustproof).

Electrical system

It is mainly composed of high-voltage jumper or high-voltage harness, low-voltage harness and relay. The high-voltage harness can be regarded as the "arterial blood vessel" of the battery pack, which continuously transmits the power of the heart of the power battery system to all the needed components, while the low-voltage harness can be regarded as the "neural network" of the battery pack, which transmits the detection signal and the control signal in real time.

Thermal management system

There are four main categories: air cooling, water cooling, liquid cooling and phase change materials. Taking the water cooling system as an example, the thermal management system is mainly composed of cooling plate, cooling water pipe, heat insulation pad and heat conduction pad. The thermal management system is equivalent to packaging an air conditioner for the battery. The process of battery charging and discharging is actually the process of chemical reaction, which will release a lot of heat. The battery needs a thermal management system to take away heat and keep the battery in a reasonable working temperature range to improve its life and reliability.

BMS battery management system

BMS can be regarded as the "brain" of battery pack, which is mainly composed of CMU and BMU. The battery management system can detect and adjust the working state of the battery in time to ensure the working safety of the battery, thus increasing the cruising range and prolonging the service life.

CMU monomer monitoring unit is responsible for measuring battery voltage, current, temperature and other parameters, as well as balancing and other functions. When CMU measures these data, it transmits the data to BMU through the battery "neural network" mentioned above.

BMU battery management unit. Be responsible for evaluating the data sent by CMU. If the data is abnormal, protect the battery, send a request to reduce the current, or cut off the charging and discharging path to prevent the battery from exceeding the allowable use conditions, and also manage the battery's power and temperature. According to the previously designed control strategy, the parameters and states that need to be alarmed are judged, and the alarm is sent to the vehicle controller and finally conveyed to the driver.

Take Tesla for example. Tesla has the world's leading battery management technology, model? S*** adopts 7 104 18650 batteries, every 74 batteries are connected in parallel to form a battery pack, every 6 batteries are connected in series to form a 1 battery pack, and 16 batteries are connected in series to form Tesla's whole power battery. BMS monitors each checkpoint in real time, cools each battery in time, reduces the temperature difference between them and prevents spontaneous combustion. However, Tesla can monitor every cell, but it can't solve the situation of burning and explosion under the extreme damage of battery pack impact, which is also the reason why two Tesla cars in the United States caught fire this year.

1.2 electric drive system

The electric drive system is mainly composed of transmission mechanism, motor and inverter.

gear

At present, the transmission mechanism of electric vehicles at home and abroad is single-machine deceleration, that is, there is no clutch and no speed change. In the future, electric vehicle enterprises will increase the complexity of transmission mechanism and reduce the demand for motor and motor rheostat, that is, improve performance and reduce costs.

electrical machinery

There are two main types of motors used in new energy vehicles: permanent magnet synchronous motors and asynchronous motors. Permanent magnet motor, as its name implies, is a motor with a magnetic core. Permanent magnet synchronous motor (PMSM) is a synchronous motor that is excited by permanent magnets to generate synchronous rotating magnetic field. Permanent magnet as rotor generates rotating magnetic field, and three-phase stator winding induces three-phase symmetrical current through armature reaction under the action of rotating magnetic field.

As a permanent magnet, the rotor of a permanent magnet synchronous motor generates a magnetic field in a fixed direction, and the rotating magnetic field of the stator "drags" the rotor magnetic field (rotor) to rotate, so the speed of the rotor must be equal to the synchronous speed of the stator, so it is called a "synchronous motor". The rotor speed of permanent magnet synchronous motor is always consistent with the current frequency of stator winding. Therefore, the speed of electric vehicle can be controlled by controlling the input current frequency of motor stator winding.

The biggest difference between it and asynchronous motor lies in the unique structure of rotor and high-quality permanent magnetic pole. Asynchronous motor and permanent magnet motor are similar in structure, except that there is no magnetic core, and there is no magnetic field without electricity, so the magnetic field can only be generated by consuming a lot of electricity. Its maximum speed can reach 15000 rpm, and it is a high-speed motor, which generates great power at high speed.

Compared with permanent magnet synchronous motor, asynchronous motor has the advantages of low cost, strong overload capacity, simple structure, convenient manufacture and good reliability, but it is bulky and consumes a lot of energy. Under the condition of the same battery capacity and the same car condition, the car with synchronous motor has more cruising range than the car with asynchronous motor. Like Tesla model? s？ P85D, the mass of the whole vehicle is more than 2. 1 ton, because the battery utilization efficiency of asynchronous motor is low and the volume is large, so more batteries need to be installed.

Although the cost of permanent magnet synchronous motor is slightly higher than that of asynchronous motor, it has compact structure, small volume and high power density. Limited battery capacity can achieve the best cruising range. Chevrolet Chevrolet patrol car officially uses permanent magnet synchronous motor.

transverter

Inverter is a device that converts direct current into alternating current. If the inverter of the electric vehicle can support higher voltage, then the corresponding voltage charging current is larger and the power is larger, which means that the charging power can be amplified in equal proportion when charging with the same current, that is, the charging time will be shortened. If the supporting voltage of the inverter is increased, the heat generated by the inverter will increase when charging, so it is necessary to solve the heat dissipation problem of IGBT module in the inverter, which is the key problem to improve the charging efficiency.

1.3? electronic control system

The electronic control system is the nerve center connecting the motor and the battery, which is mainly used for dynamic monitoring of the whole vehicle and timely feedback and adjustment of various technical parameters. It is divided into two categories: motor control system and BMS, in which BMS is introduced in detail in the first half.

As the commander-in-chief of automobile power, the core of motor control system is IGBT (insulation? The gate? Bipolar disorder? Transis, insulated gate bipolar transistor chip). The power battery of the new energy vehicle provides direct current, and the driving motor needs three-phase alternating current, which requires the motor control system to work and convert direct current into alternating current, and the key component to complete this conversion is IGBT.

Due to the limitation of space, this paper will mainly introduce the highlights of Chevrolet's power battery pack, and the motor and electric drive technology will continue to develop in the future.

2. A new intelligent water circulation temperature management system.

The matching ternary lithium battery pack in Chevrolet Changyou 4 10 version adopts the structure of 6-series aluminum extrusion tray, 1 and 96-series batteries, with a battery capacity of 52.5kWh and a lithium ternary formula of 523 (Ni-Co-Mn) and an energy density of 140Wh/kg.

The performance, life and safety of lithium-ion batteries are very sensitive to temperature. It can be said that temperature is the most prominent factor affecting the working performance of lithium-ion batteries. The temperature range of lithium-ion batteries should be kept between 25℃ and 40℃, and the temperature difference between single batteries should be less than 5℃. If the heat generated during the charging and discharging process of the battery is not properly released, the temperature of the battery will rise due to the accumulation of heat, thus greatly reducing the working efficiency of the battery. When the temperature of lithium-ion battery increases by 65438 0℃, the battery capacity will decrease by 0.2%, and when the temperature exceeds the limit operating temperature, the aging of lithium-ion battery will be accelerated.

In addition, statistics show that among the fire causes of electric vehicles, battery spontaneous combustion accounts for 22% during driving, 16% during parking and 14% during normal charging, which adds up to * * * accounting for 52%.

Overall distribution map of fire causes of electric vehicles in recent five years

In other words, the three main causes of electric vehicle fire accidents are battery spontaneous combustion, charging and car collision. The internal mechanisms corresponding to these reasons are internal short circuit, external short circuit and overcharge of the battery and its further thermal runaway.

Summary of inducement of battery thermal runaway

The trigger causes of thermal runaway are complicated, and it is generally believed that the main failure forms of thermal runaway of batteries can be classified into four categories, namely, internal short circuit, external short circuit and overcharge and overdischarge. These failures will lead to a sharp increase in battery temperature, and with the increase of battery temperature, a chain exothermic reaction will occur inside the battery.

When the battery temperature reaches 80 ~ 120? At℃, SEI? The membrane decomposes, and then the negative active material loses its protection, and the lithium metal embedded in the negative electrode reacts with the electrolyte. If the temperature continues to rise, the porous membrane of the battery will close, blocking the external short-circuit current loop and playing a certain role in self-protection. However, if the temperature continues to rise, the membrane will be at 190? Disintegration at about℃ will cause internal short circuit, release a large amount of electric energy to make the temperature rise rapidly, and then trigger the decomposition reaction between anode and electrolyte. The decomposition of the positive electrode will release a lot of heat, which will make the battery pack overheat, melt and spontaneously ignite!

Trigger mechanism of thermal runaway

Key point here: To sum up, controlling the thermal management of battery temperature is a necessary condition for building a safe new energy electric vehicle!

Fortunately, GM applied the self-developed industry-leading brand-new intelligent water circulation temperature management system of battery pack to Chevrolet cruise, creating a truly worry-free and safe car.

Brand-new intelligent water circulation temperature management system for battery pack

Liquid cooling and battery heating technology make the battery stable in an efficient and stable working range through intelligent water temperature control: in extremely cold environment, the system drives the heater to heat the battery coolant to improve the charging efficiency; In the high temperature environment, the battery core temperature is maintained in a more suitable working range (30℃-40℃ without active communication) through cooling water circulation, which further enhances the environmental adaptability of the vehicle.

The evaporation pressure of the refrigeration system is collected by the pressure sensor on the refrigerant side, and the double temperature inspection is performed by the battery coolant temperature sensor. Through the dual precise control of refrigeration, heating module and water pump, the battery temperature can be accurately monitored.

Independent battery temperature management module: through intelligent temperature control algorithm, energy consumption can be accurately distributed in real time, and the battery temperature control requirements can always be met, so as to realize the comfort control of the passenger compartment.

In addition, some studies have shown that the temperature difference between the internal temperature of the battery and the actual environmental temperature, as well as the temperature difference between the single cells in the battery pack, will adversely affect the performance, life and safety of the battery. Therefore, the internal structure, placement position, cooling conditions and other factors of the single battery can not meet the requirements of stable temperature range and uniform heat distribution during heat generation, heat transfer and heat dissipation of the whole battery pack.

The space-grade nano-temperature control material aerogel is used between the batteries of Chevrolet Changyou, and the temperature is effectively locked through nano-micropores, which greatly assists the temperature difference control and temperature isolation during a single battery. Moreover, fire blankets are laid between modules, which effectively improves the flame retardant ability, further ensures the stability of the battery itself, and provides a "hardware" foundation for preventing thermal runaway.

The battery pack is customized according to general global standards: the battery cell formula, specifications, packaging and protection all reach the highest standards in the industry. Aerospace grade aerogels are arranged between batteries, and high-performance composite heat insulation materials are arranged between modules and in the shell, covering the triple protection measures of battery cell level, module level and shell level, effectively improving the heat insulation and flame retardant performance and improving the consistency and safety of the battery pack.

Space-grade aerogels are arranged between batteries.

The application of a complete set of intelligent water circulation temperature management system and aerospace-grade nano temperature control material aerogel also improves the charging speed, and can realize fast charging of direct current 10 minute, vehicle running 100KM (limited working condition: 25℃+4%SOC charging, new battery).

3. Is the battery pack safer and more acceptable?

At the same time, we can also find that short-circuit fire after collision accounts for 14%, which is also the biggest concern of ordinary consumers about the safety of new energy electric vehicles compared with traditional fuel vehicles.

That is, what is the safety performance of the battery after the collision of new energy electric vehicles, and whether the battery will spontaneously ignite due to extrusion deformation and explosion!

Fire after collision 14%

The answer is of course no!

Extreme conditions such as collision, fire, waterproof and extreme temperature have been considered at the beginning of the design of power batteries. To cope with emergencies.

Image from Zhihu User @ Fargo Please come in.

Take the basic process of square battery manufacturing as an example. In design, testing and verification, every stage has to go through extreme security considerations. In order to discharge the gas and pressure generated inside the battery when it is squeezed by external force in the case of collision or bottom dragging, an explosion-proof valve is designed for the square battery core. Similarly, in order to protect the safety of vehicles and batteries in extreme cases through internal and external shunt during collision, the battery bottom case is specially designed with a certain amount of collapse.

So what is the battery safety performance of Chevrolet? How can GM engineers ensure the safety of batteries?

There are six national standards for power batteries in China: GB/T? 3 1484-20 15。 Six national standards test battery cells, battery modules and battery packs/systems from three dimensions: mechanical safety, environmental safety and electrical safety, and give relevant judgment standards. And the difficulty of the national standard test is also extremely harsh: in the extreme temperature test, the national standard requires that the battery of the electric vehicle can work normally and stably under the drastic changes from MINUS 20 degrees to above zero 50 degrees and close to 70 degrees. In the extrusion and acupuncture tests, it is required that the battery can survive when it is squeezed by an external force of 10 ton; When the 1 cm-thick 1 steel needle passes through the geometric center, the battery is in a short-circuit state and will not explode or catch fire.

Nail penetration test

Extrusion test, extrusion pressure 100KN.

Compared with the national standard, GM and its parts manufacturers have stricter requirements for battery testing, and the types and difficulties far exceed the national standard. General Motors has carried out 13 extreme safety tests on the lithium battery pack on Chevrolet cruise: seven extreme safety tests, such as breakdown (module-level thermal diffusion test exceeding the national standard), extrusion, soaking, fire, overcharge, overdischarge and short circuit, such as overtemperature, collision, vibration, temperature shock, damp-heat shock and salt spray, have reached IP67 level.

Taking fire detection as an example, the national standard requires that batteries burn 130 seconds without explosion. On the battery pack patrolled by Chevrolet, this standard has been raised to the point where it can work normally when burning 130 seconds, and there is no explosion danger when burning 1 hour.

Ignition point determination

It is worth mentioning that GM has innovatively increased the longitudinal beam, drop and side column collision experiments higher than the requirements of the regulations at the vehicle level to ensure the safety and stability of daily use.

In addition, Chevrolet's battery pack design has a triple physical protection structure. Because the energy absorption space at the front end of pure electric vehicles is reduced compared with traditional vehicles, the installation position of high-rigidity and large-mass batteries is very different from that of traditional engines. At the same time, the special parts of electric vehicles that are different from traditional fuel vehicles also put forward higher requirements for their collision safety performance.

Therefore, Chevrolet Changyou added a lower anti-collision beam (high-strength steel), a double frame structure design (ultra-high-strength steel) and a plurality of transverse through beam structure designs (ultra-high-strength steel) made of high-strength and ultra-high-strength steel to the frame, which provided sufficient protection for the high-voltage battery pack, thus avoiding the collision and extrusion of the high-voltage battery pack during the collision. Compared with the traditional five-star refueling vehicle, the first two transverse penetrating beams and the protective beams around the battery pack are added, which meets all-round safety requirements.

Conclusion:

Battery safety is an important index for the safety of new energy electric vehicles. Only batteries designed, tested and manufactured according to the highest standards can be called safe luxury cars.

Especially for the three electric systems, high-standard design-manufacturing-inspection-acceptance is more important in pure electric vehicle products. At the critical stage of building consumer confidence, too many enterprises' semi-finished products have accidents during use, leaving many hidden dangers. This is the seed planted by the immaturity of the whole process system, which will even have an impact on the whole market.

For GM Chevrolet, a brand with a complete and mature system, although it is a bit slow on the road of localization of electric vehicle products, in my opinion, it has already possessed foolproof ability in core technology.

This article comes from car home, the author of the car manufacturer, and does not represent car home's position.