Briefly describe the battery management system of Ningde era and BYD

If the battery is likened to the heart of the human body, the module and the battery pack are like a strong body, then the entire power battery system needs to run smoothly, and a brain that governs the body is needed, that is BMS.

The Battery Management System (BMS) is an important link between battery and electric vehicles. Its precise control and management escort the perfect application of the battery.

“The dragon gives birth to nine children, all different” Even if two single cells produced in the same batch are not consistent in performance due to production process errors and usage environments, such inconsistencies during use. It will gradually expand, and there may be dangers of overcharging, over discharge and local overheating, which will seriously affect the service life and safety of the battery pack.

At this time, you need BMS to show your talents.

So the question is coming, what is the main thing for BMS?

The function of the battery management system BMS

1. Accurately estimate the state of charge of the power battery pack

Accurately estimate the state of charge (State SOC) of the power battery pack, that is, the remaining battery power, to ensure that the SOC is maintained within a reasonable range, to prevent damage to the battery due to overcharge or over discharge, thereby predicting the hybrid vehicle reserve at any time. How much energy is left in the battery or the state of charge of the energy storage battery?

2. Dynamically monitor the working status of the power battery pack

During the charging and discharging process of the battery, the terminal voltage and temperature, the charging and discharging current and the total voltage of the battery pack of each battery in the power battery pack are collected in real time to prevent overcharging or over discharging of the battery.

At the same time, the battery condition can be given in time, and the problematic battery can be selected to maintain the reliability and high efficiency of the entire battery operation, and the realization of the remaining battery estimation model becomes possible. In addition, it is necessary to establish a usage history file for each battery to provide data for further optimization and development of new types of electricity, chargers, motors, etc., and to provide a basis for offline analysis system failure.

3, the balance between the cells

That is, the single battery is uniformly charged, so that each battery in the battery pack reaches a state of equalization and consistency. Equilibrium technology is the key technology of a battery energy management system that the world is currently researching and developing.

Regarding the function of BMS, the classification in the industry is different. However, from the user's point of view, it can be roughly divided into two major functions - "battery check" and "security guard".

Instant medical check to accurately grasp battery status

Instant "physical examination" refers to battery data collection and status assessment.

Data acquisition can be simply understood as a routine "physical examination" for the battery; during the charging and discharging process, the terminal voltage, temperature, charge and discharge current and total voltage of each battery in the battery pack are collected in real time to prevent the battery from being overcharged or Over discharge phenomenon.

This "physical examination" is online, continuous, and uninterrupted. When the data is found to be abnormal during the process, the battery condition can be queried in time, and the problematic battery is selected to maintain the reliability and efficiency of the entire battery operation.

Ningde era masters the industry's leading high-precision measurement technology, the total flow total pressure accuracy can reach five thousandths; the sampling data has high precision, and the judgment and correction are made in time by real-time understanding of the real working state of the battery.

After the "physical examination" is over, it will enter the stage of analysis, diagnosis and calculation, and then generate a "physical examination report", which can be understood as the state assessment of the battery.

At this time, we need to understand the common terminology of an industry - SOC.

What is SOC?

The state of charge (State SOC) of the battery pack is the remaining battery power. The SOC is the basis for judging a series of faults such as overcharging and over discharging of the battery. Accurate estimation of the SOC can prevent the battery from overcharging and over discharging, prolonging the service life of the battery, thereby improving the utilization of the battery.

In fact, in addition to the SOC estimation, there are SOH (State of Health), SOP (State of Power), the user can see the data through the on-board instrument display, thus confirming the battery's working and functional status. Based on this, on the basis of protecting the battery, the potential is maximized and the driving experience is greatly improved.

Therefore, the accuracy of data estimation such as SOC is particularly important. The consequences of the estimation are not allowed. It may be that the car breaks down and does not match the expected mileage.

When the vehicle is undergoing a fast charge experiment, the SOC shows 52%

For example, a vehicle with a cruising range of 400 kilometers at full power is on the road. If the estimation is accurate, when the SOC is displayed as 10%, the mileage that may be traveled is 40 kilometers; if the estimation is not accurate, the SOC reaches 15% the user's mileage is 60 kilometers, in fact, after driving for 40 kilometers, already dead. Obviously, for the user, this situation is very bad.

Estimating the state of the battery requires a series of complicated calculations. In the Ningde era, the precise core algorithm was mastered. The estimation method based on battery parameters effectively eliminated the influence of cumulative error and the estimation was more accurate. NCM estimates accuracy at 3% and LFP at around 5%.

"Security Guard" protects battery and personal safety

BMS has another core function, which is “security guard”; it can be simply understood as the role of protection, mainly in the protection of batteries and high voltage safety.

First of all, due to the complexity of the internal structure of the battery, battery abuse or damage will affect the battery life and safety. In severe cases, the battery will be out of control and cause a safety accident. BMS's battery protection functions are: on the one hand, real-time monitoring of battery status; on the other hand, communication with external systems such as vehicles and chargers to timely and effectively control the charging and discharging process to prevent dangerous accidents occur.

At present, the Ningde era has realized the all-round protection of the BMS on the charging, discharging and equalization process of the battery, and has achieved the prevention, detection, process control and degradation of the fault.

Engineers test the prototype winter in Heihe at -30 °C

Secondly, the battery system voltage can reach 300-500V, far exceeding the human body safety voltage, the risk is extremely high, and the high-voltage safety protection is escorted. By detecting the insulation resistance, high voltage interlock and relay status, BMS detects and controls the risk of high voltage leakage that may occur, thus protecting the safety of drivers, passengers and maintenance personnel.

Engineers perform high voltage testing at the OEM

When it comes to security, you have to mention functional security. Functional safety is at the heart of BMS security development to prevent, detect and control the dangers of power battery systems caused by BMS and other E/E component failures. Ningde era has always been the world's first safe battery development goal. The industry entered the functional safety development field earlier, and is the first power battery enterprise to develop BMS products with ASILD functional safety objectives.

What is the ASILD rating?

The international safety standard ISO26262 divides the safety requirements into safety levels (AIL) from A to D according to the degree of safety risk. The D level is the highest level and is the most demanding safety requirement, which means that the functional safety development process and technical requirements are more Strict and corresponding development costs increase and the development cycle is extended.

Random hardware failure rate requirements for products with different safety levels

The ASILD rating requires a product safety target failure rate of less than 10^-8/h, which means that one vehicle is assumed to operate for 4 hours per day, and it takes 70,000 years to run before a functional failure that violates safety objectives. And such a low probability of failure rate can be compared with the safety design requirements of the aircraft.

Ningde Times has provided or cooperated with many domestic and foreign customers such as BMW, Volkswagen, Peugeot Citroen and Great Wall to develop BMS products that meet functional safety requirements, and has been unanimously recognized by customers and third-party accreditation bodies. While accumulating rich project experience, the Ningde era has gradually formed a huge advantage in the functional safety of BMS products.

In fact, the function of BMS is far more than these. It can be described as "small body, great wisdom." The battery management system cooperates with the battery pack to provide a powerful power battery system for the entire vehicle. In the future, the Ningde era will also go hand in hand and embark on a higher and farther journey!

Decrypt the BYD battery management system

First of all, let's talk about the battery of Tang and Qin. The model should be the same, but the number of batteries in Qin's battery pack is relatively small, the capacity is 13 degrees, and the number of Tang is more, 18 degrees. The individual batteries are BYD's own lithium iron phosphate batteries, rated at 3.2V and capacity 26AH. Why is it not a relatively recent ternary lithium battery? The reason is as follows:

lithium iron phosphate battery has better life and safety, and is more suitable for the use of plug-in hybrid vehicles.

The battery cell is probably like this, but this should be on the bus, because the electricity reserves are as high as 120AH, ours only 26AH, but roughly the same, are cuboids.

Tang's battery pack is located in the middle of the chassis and is relatively large in size and weight. The advantage of being placed on the chassis is to reduce the center of gravity of the vehicle without affecting the trunk space. Disadvantages, the requirements for water discharge and anti-collision are relatively high, daily use should pay attention to this piece not to soak, do not bump.

This is Qin's battery pack, located in the back seat, before the trunk. Advantages: The performance of water release and anti-collision is very good. Disadvantages: the center of gravity is relatively high, affecting the trunk space, and Tang is just opposite..

The connection method is series connection (all cells are connected in series). The battery in series is as shown in the figure below. The image is similar to the flashlight we used before. Several batteries are connected end to end.

In this connection mode, each cell discharges with the same current and discharges at the same current. When charging, the same current is charged, and the single cell cannot be charged and discharged without the aid of the equalization system. Moreover, when a battery core is full, it is necessary to stop charging the entire battery pack. Otherwise, the battery core will be overcharged and damaged, and when one battery cell is emptied, the entire battery pack will stop discharging, otherwise the battery core will pass. Put damage.

Remember what are the requirements for flashlights? Right, old and new batteries can't be mixed, which means that batteries with and without electricity can't be mixed. Back to Tang and Qin's battery pack, the top is a schematic, and several batteries are selected. Under normal circumstances, their power storage should be exactly the same, filled together, and emptied together. If it has been cycled like this, then there will be no problems at the beginning of the article.

In fact, after using the battery pack for a period of time, there will be a difference in the storage capacity of each battery. There are many reasons for the difference. For example, the battery itself has inconsistent capacity, or the internal resistance is inconsistent, the operating temperature is inconsistent, etc., which will cause the discharge. There is a difference in capacity. When the power storage capacity of each battery is inconsistent, the following picture will appear:

On the surface, there is only one battery that loses a little power. There are so many batteries, should there be any impact? Let's continue to look down, what happens when this battery pack is discharged:

The entire battery pack releases 80% of the power, and at this time, the battery that was originally dissatisfied is empty, and the battery pack will stop discharging. If the storage capacity of this battery pack is 10 degrees, then in the case of fullness, the unbalanced battery pack discharges 80%, that is, 8 degrees, and it cannot be discharged. On the surface, only 5% of the power is missing, but it leads to 20 % of capacity is not available. In the case where there are only 4 batteries compared, if there are more than 200, it is conceivable how much the impact is.

So what happens if there is an imbalance? This requires the equalization module of the battery management system. Tang and Qin's equalization module uses a passive equalization method, that is, a capacitor with a higher voltage is discharged through a shunt resistor to achieve the same voltage as other cells. That's it:

Each cell has a resistor that is individually controlled by the battery management system. When needed, the circuit of this resistor is turned on to discharge the cell. After a certain amount of time, this unbalanced battery pack becomes like this:

The battery capacity is the same, the recharge can be full, the discharge is empty, everything is back to normal, the capacity is back, and the battery life is back! It sounds beautiful, is it? Then why can't many cars achieve this effect? ?

First of all, this discharge process is very slow! The current can reach 10A or more (10000ma) during the charging process, and this discharge? It is understood that the maximum current allowed by this discharge resistor is 30ma~ in the case where the equalization system is always in the best equilibrium state. Under the same time, it takes about 100 hours to balance the difference in electricity.

Second, the equalization system does not always work at its best. To have a good working condition, the system needs to know which battery is needed to be discharged and how much power to put. This process is not complete with any power.

This is a graph of the discharge of a lithium iron phosphate battery. It can be seen that the voltage difference is very small at 15% or more. At this time, it is very difficult or even impossible to find which battery core needs to be discharged and how much. Therefore, in order for the equalization system to work efficiently, it is necessary to use the battery in less than 15% in real time.

Then fully charged, let the car into equilibrium, this time the equilibrium efficiency is the highest, unless the car, or it is recommended to wait until the end of the equilibrium (that is, the dashboard is completely extinguished). In the case of an unbalanced battery pack, it takes about 20 hours for a single equalization. You can calculate how many cycles you need based on the lack of power in your battery pack.

This also leads to another problem: after the end of the equilibrium, a little bit of electricity, then full, the vehicle will once again enter equilibrium, this time should not be counted in the effective equilibrium? According to the experience of the landlord, this equilibrium is almost invalid.

Because Tang and Qin's battery packs are not balanced, most of them are too low voltage of one or two batteries, and it is necessary to discharge another large number of batteries. In the case of low power, the remaining cells can be correctly marked. Under high power, the system will only mark one cell with the highest voltage when it is full. It is one. It is conceivable that the efficiency is almost negligible.

Let's talk about what kind of battery is ok, what kind of problem is there. Here, 14 Qin's DCT software battery monitoring module was borrowed to display data. Don does not support this, but the principle of the battery pack is the same.

When many people check the battery, they find that the lowest voltage battery is only 2.6-2.8V. It feels that there is a problem with this battery, and then the 4S shop is required to be replaced. The 4S sets the manufacturer's form and gives a normal reply. The customer will feel the manufacturer. In fact, a single cell voltage lower is normal.

The ideal situation is that 5% of the power is lower than 3V for all voltage cells, so that all the battery packs are discharged. Of course, such a battery pack is almost non-existent, and it requires very uniformity of all the batteries. In general, the basis for judging the battery pack condition is that in the case of 5%, the lowest cell voltage is lower than 3V, and the highest voltage cell voltage is lower than 3.15V (discharge to 5% of the instantaneous voltage, storage After a while, the voltage will rise and it will not rise.

The battery manufacturer has its own standard. If the replacement conditions are met, you can choose to replace it. However, the landlord recommends using the correct equalization method for 100 hours. If the effect is not obvious, change it. Because the replacement of the battery and the original has some attenuation of the battery is very difficult to match.

If there is a problem with a battery and the actual capacity is reduced, then no matter how hard the equalization system works, it will not help. So how do you judge the battery problem?

The voltage caused by the equalization problem is inconsistent. When the voltage is 5%, the lowest voltage cell is the same as the lowest voltage cell at 100%. The battery problem is caused by the lowest voltage cell at 5%, but the voltage is higher or even higher at 100%. If your battery pack is like this, then there is no other way to replace the problematic battery. Core!

Insufficient charge and insufficient battery life: there is a problem with battery pack equalization or a problem with a battery. The solution is to first determine which situation, the corresponding treatment comments have been introduced in the previous article.

Charging and Jumping: The battery pack is at a certain percentage (such as 96%) when charging, and directly reaches 100% without going through the percentage. The reason is that the system's mark for the battery pack capacity is greater than the actual battery pack's capacity. When charging to this percentage, the voltage of the cell has reached the voltage to terminate charging. Therefore, the system stops charging, and at the same time, it is determined that the power is 100% at this time, and the cause of this problem is also insufficient charging.

Low power drops fast: because of the discharge characteristics of the lithium iron phosphate battery, the voltage changes over a long platform in the middle are so low that the system can only estimate the remaining charge. When the cell's remaining charge reaches 15%(at which point the corresponding cell voltage is about 3.18v), the voltage suddenly drops.

Tang and Qin's battery management system will re-estimate the remaining battery capacity when the battery reaches this voltage. If the remaining power is displayed at 30%, and the system is re-estimated and considered to be only 15%, then the management system will increase the speed of the battery display, the result is that the original 1% can run 800 meters, and at this time can only run 400 meters.

The dilemma of domestic battery management system BMS

The development of new energy vehicles is not smooth sailing. In the past two years, with the large-scale promotion and use of new energy vehicles, we have also heard a lot of "scandals" about new energy vehicles: spontaneous combustion, false mileage, etc., and why These usage problems arise? The lack of a battery management system or the use of a poor quality immature battery management system is the main cause. In fact, the safety of new energy vehicles has always been one of the key tasks of the government and the automotive industry.

Not long ago, the four ministries and commissions including the Ministry of Science and Technology, the Ministry of Finance, the Ministry of Industry and Information Technology and the National Development and Reform Commission have jointly issued a “safety order” for the demonstration and promotion of new energy vehicles (the “Letter on Strengthening the Safety Management of Energy Saving and New Energy Vehicles Demonstration and Promotion”), emphasizing "The plug-in hybrid electric vehicle and pure electric vehicle that are put into demonstration operation should all be equipped with real-time monitoring system (BMS) for vehicle operation technology status, especially to strengthen the self-ignition of power battery and fuel cell electric vehicle. It is not a battery management system installed, you can rest easy, for example: in terms of safety, accuracy, life, discharge capacity, etc., the single battery can be charged and discharged 2000 times, after the battery pack may be only 1000 times, if equipped with immature BMS, It is impossible to accurately monitor the battery charge and discharge status in real time, which is very likely to cause excessive local power consumption of the battery core, generate local heat, and the information cannot be transmitted to the driver, which easily leads to spontaneous combustion of the battery.

The industry believes that the installation of excellent battery management BMS can effectively improve the utilization of the battery, prevent overcharging and over discharging of the battery, and extend the battery life, monitor the operating state of the battery pack and each battery core, effectively prevent the battery The group is self-igniting, and in the event of an emergency, the driver is alerted to the accident in advance to win time for security.

The future of new energy vehicles and battery management systems

Began in the early 21st century, China's new energy automobile industry development decade, however, so far as people thirst for environmental protection and renewable energy, new energy vehicles just ushered in the development opportunity, then open the floodgates, over a long period in the future, new energy vehicles will be as a challenger to occupy originally belongs to the mass marketing of traditional fuel vehicle, and as a result of the need of social development, seize the market share, can be expected.

While looking forward to the rapid development of new energy vehicles, we must clearly understand that the development of technology is the basis for the development of the industry, and stable, efficient, safe and reliable products are the embodiment of technology. We must know that the current domestic the new energy auto industry is not friendly. The frequent spontaneous combustion events and false cruising range of electric vehicles have exposed the imperfections in the design, testing and production standards of new energy battery packs and battery management systems in China.

The lack of technical parameters and standards, and no authoritative organization to conduct authoritative testing of BMS produced by manufacturers is the current dilemma of the domestic BMS market, resulting in the unevenness of BMS products and difficult to large area promotion.

The page contains the contents of the machine translation.