The Battery Management System (BMS) is a crucial link between batteries and electric vehicles, and its precise control and management ensure the perfect application of batteries.
"The dragon has nine sons, each different from the other." Even two individual battery cells produced in the same batch cannot have completely identical performance due to manufacturing process errors, differences in usage environment, etc. During use, this inconsistency will gradually increase, which may lead to the dangers of overcharging, over-discharging, and local overheating, and in severe cases, affect the lifespan and safety of the battery pack.
This is where BMS comes in handy.
So the question is, what is the most important function of a BMS?
Functions and uses of Battery Management System (BMS)
1. Accurately estimate the state of charge of power lithium battery packs
Accurately estimate the State of Charge (SOC) of the power lithium battery pack, i.e. the remaining battery capacity, and ensure that the SOC is maintained within a reasonable range to prevent damage to the battery due to overcharging or over-discharging. This allows for real-time forecasting of how much energy or the state of charge of the hybrid vehicle's energy storage battery is remaining.
2. Dynamically monitor the operating status of the power lithium battery pack.
During the charging and discharging process, the terminal voltage and temperature of each battery in the power lithium battery pack, the charging and discharging current, and the total voltage of the battery pack are collected in real time to prevent overcharging or over-discharging of the battery.
Simultaneously, it can promptly provide battery status information, identify problematic batteries, maintain the reliability and efficiency of the entire battery pack, and make the implementation of the remaining power estimation model possible. In addition, it is necessary to establish a usage history record for each battery to provide supply data for further optimization and development of new batteries, chargers, motors, etc., and to provide a basis for offline system fault analysis.
3. Balancing between individual cells
This refers to the equalization charging of individual cells, ensuring that all cells in the battery pack reach a balanced and consistent state. Equalization technology is a key technology in battery energy management systems that is currently being researched and developed worldwide.
There are various ways to classify the functions of a Battery Management System (BMS) within the industry. However, from a user's perspective, they can be broadly divided into two main functions: "Battery Health Check" and "Safety Guardian".
Immediate medical check-up
Accurately monitor battery status
Real-time "health check" refers to battery data collection and status assessment.
Data acquisition can be simply understood as performing a routine "physical examination" on the battery; during the charging and discharging process, the terminal voltage, temperature, charging and discharging current and total voltage of each battery in the battery pack are collected in real time to prevent the battery from overcharging or over-discharging.
This "health check" is online, continuous, and uninterrupted. When data anomalies are detected during the process, the status of the corresponding battery can be queried in a timely manner, and the problematic battery can be selected, thereby maintaining the reliability and efficiency of the entire battery pack.
CATL possesses industry-leading high-precision measurement technology, with total current and total pressure accuracy reaching 0.5%. The sampling data has very high accuracy, enabling timely judgment and correction by understanding the actual working status of the battery in real time.
After the "physical examination" is completed, the analysis, diagnosis, and calculation stages will begin, and then a "physical examination report" will be generated. This process can be understood as a battery status assessment.
At this point, we need to understand a commonly used industry term – SOC.
What is SOC?
The State of Charge (SOC) of a battery pack is the remaining charge of the battery. SOC is the basis for judging a series of faults such as overcharging and over-discharging. Accurate SOC estimation can prevent overcharging and over-discharging, extend battery life, and thus improve battery utilization.
In addition to SOC estimation, there are also SOH (State of Health) and SOP (State of Power). Users can view these data on the vehicle's instrument panel to confirm the battery's working and functional status. Based on this, the battery's potential can be maximized while protecting it, greatly enhancing the driving experience.
Therefore, the accuracy of data estimations such as SOC is particularly important. Inaccurate estimations can lead to consequences such as car breakdowns or discrepancies between the estimated mileage and actual driving distance.
The vehicle's SOC showed 52% during a fast charging test.
For example, a vehicle with a range of 400 kilometers on a full charge is driving on the road. If the estimate is accurate, when the State of Charge (SOC) shows 10%, it may still be able to travel 40 kilometers. However, if the estimate is inaccurate, and the SOC reaches 15%, the user may think they have 60 kilometers of range, but in reality, they may run out of power after only 40 kilometers. Obviously, this situation is terrible for the user.
Estimating battery state involves a series of complex calculations. CATL has mastered a precise core algorithm that effectively eliminates the impact of accumulated errors through an estimation method based on battery parameters, resulting in more accurate estimates. The accuracy of NCM estimation is around 3%, and LFP is around 5%.
"Security Guardian"
Protect the battery and personal safety
Another core function of BMS is "safety guardian"; it can be simply understood as protection, mainly in the protection of battery and high voltage safety.
First, due to the complexity of the battery's internal structure, battery abuse or damage can affect battery life and safety, and in severe cases, cause thermal runaway, leading to safety accidents. The Battery Management System (BMS) protects the battery by: 1) monitoring its status in real time; and 2) communicating with external systems such as the vehicle and charger to effectively control the charging and discharging process, thereby preventing dangerous accidents.
Currently, CATL has achieved comprehensive protection of the battery charging, discharging and equalization process through its BMS, enabling early prevention of faults, detection of faults, process control and degradation of impact.
Engineers conducted winter testing of the prototype vehicle in Heihe at -30°C.
Secondly, the battery system voltage can reach 300-500V, far exceeding the safe voltage for the human body, posing a significant risk. High-voltage safety protection is therefore essential. The BMS detects and controls potential high-voltage leakage risks by monitoring insulation resistance, high-voltage interlocks, and relay status, thereby protecting the personal safety of drivers, passengers, and maintenance personnel.
Engineers conduct high-pressure tests at the vehicle assembly plant
When discussing safety, functional safety is paramount. Functional safety is the core of BMS safety development, aiming to prevent, detect, and control hazards to power lithium battery systems caused by BMS and other E/E component failures. CATL has always aimed to produce leading-edge safe batteries and was one of the earliest companies in the industry to enter the field of functional safety development. It was also the first power lithium battery company to develop a BMS product with ASILD functional safety objectives.
What is the ASILD rating?
The international safety standard ISO 26262 classifies safety requirements into safety levels (Automotive Safety Integrity Level, D) from A to D based on the degree of safety risk. Level D is the highest level and represents the most stringent safety requirements, which means that the functional safety development process and technical requirements are more stringent, the corresponding development costs are increased, and the development cycle is extended.
Random hardware failure rate requirements for products with different security levels
The ASILD rating requires a product safety target failure rate of less than 10^-8/h, meaning that assuming a vehicle operates for 4 hours a day, it would take 70,000 years for it to experience a functional failure that violates the safety target once. Such a low failure rate is comparable to the safety design requirements of aircraft.
CATL has supplied or co-developed BMS products that meet functional safety requirements for numerous domestic and international clients, including BMW, Volkswagen, Peugeot Citroën, and Great Wall Motors, earning unanimous recognition from clients and third-party review bodies. While accumulating rich experience in related projects, CATL has gradually developed a significant advantage in the functional safety of its BMS products.
In fact, the BMS's functions go far beyond these, truly embodying the saying "small in size, big in intelligence." The battery management system works in conjunction with the battery pack to supply the entire vehicle with a powerful lithium-ion battery system. In the future, CATL will continue to work hand in hand with its partners to embark on a journey to greater heights!
Decoding BYD's Battery Management System
First, let's talk about the batteries in the Tang and Qin. The model numbers should be the same, but the Qin's battery pack has fewer cells and a capacity of 13 kWh, while the Tang's has more, at 18 kWh. Individual cells are BYD's own lithium iron phosphate batteries, with a rated voltage of 3.2V and a capacity of 26AH. Why not the more recently popular ternary lithium batteries? The reason is shown in the following diagram:
Lithium iron phosphate batteries have better lifespan and safety, making them more suitable for plug-in hybrid vehicles.
A single battery cell looks something like this, but this one is probably from a bus because it has a capacity of 120AH, while ours is only 26AH. However, they are roughly the same, both being cuboid.
The Tang's battery pack is located in the middle of the chassis, and it's quite large and heavy. The advantage of placing it in the chassis is that it lowers the vehicle's center of gravity without affecting the trunk space. The disadvantage is that it requires higher protection against water and impacts; care must be taken to avoid immersion in water and damage during daily use.
This is the Qin's battery pack, located behind the rear seats and in front of the trunk. Advantages: Excellent water resistance and impact protection. Disadvantages: Relatively high center of gravity, affecting trunk space, which is the opposite of the Tang's.
The connection method is series connection (all battery cells are connected in series). The series connection of the batteries is shown in the figure below. To put it more vividly, it is similar to the flashlight we used to use, where several batteries are connected end to end.
In this connection method, each cell uses the same current to discharge and charge, making it impossible to charge and discharge individual cells without a balancing system. Furthermore, when one cell is fully charged, charging of the entire battery pack must stop to prevent overcharging and damage; conversely, when one cell is fully discharged, the entire battery pack must stop discharging to prevent over-discharging and damage.
Remember the requirements for flashlights? That's right, new and old batteries cannot be mixed; that is, batteries with power and batteries without power cannot be used together. Returning to the Tang and Qin battery packs, the diagram above shows several selected cells. Under normal circumstances, their stored capacity should be exactly the same; they should be fully charged and discharged together. If this cycle continues, then the various problems mentioned at the beginning of the article will not occur.
In fact, after a battery pack has been used for a period of time, differences in the amount of charge stored in each cell will occur. There are many reasons for this, such as inconsistent battery capacities, inconsistent internal resistance, or inconsistent operating temperatures, all of which can lead to differences in discharge capacity. When the amount of charge stored in each cell is inconsistent, the following situation will occur:
On the surface, it seems like only one cell lost a little charge. With so many cells in total, it shouldn't have any impact, right? Let's continue to see what happens when this battery pack discharges:
The entire battery pack has discharged 80% of its capacity. At this point, the previously partially charged cells are now empty, and the battery pack must stop discharging. If the battery pack has a capacity of 10 kWh, then when fully charged, this unevenly distributed battery pack, after discharging 80% (8 kWh), can no longer discharge. Superficially, it only appears to be a 5% capacity loss, but it results in 20% of the capacity being unusable. This is based on a comparison of only 4 cells; imagine the impact if there were over 200 cells.
So what happens if an imbalance occurs? This is where the battery management system's balancing module comes in. The Tang and Qin models use a passive balancing method, meaning that a bypass resistor discharges the higher-voltage cells until they reach the same voltage as the other cells. That's it:
Each battery cell has a resistor individually controlled by the battery management system. When needed, the circuit of this resistor is connected to discharge the cell. After a certain period of time, this unbalanced battery pack becomes like this:
Once the battery cells have the same capacity, they can all be fully charged when recharged and fully discharged when discharged, restoring everything to normal. The capacity is back, and the range is back too! Sounds great, right? So why do many cars fail to achieve this?
First, the discharge process is extremely slow! The charging current can reach over 10A (10000mA), but the discharge process? It's understood that the maximum current allowed by the discharge resistor is only 30mA. Even with the equalization system in optimal balance, it would take approximately 100 hours to equalize a difference of one kilowatt-hour!
Secondly, the balancing system does not always operate at its optimal state. For it to function well, the system needs to know which cell needs to be discharged and how much charge needs to be discharged. This process cannot be accomplished with just any amount of charge.
This is a discharge curve of a lithium iron phosphate battery. As you can see, when the battery level is above 15%, the voltage difference is very small. At this point, determining which cell should discharge and how much is extremely difficult, if not impossible. Therefore, to ensure the balancing system operates efficiently, the battery level must be kept below 15% in real time.
Then fully charge the battery and allow the car to enter a balancing state. This balancing process is most efficient. Unless you plan to use the car, it's recommended to wait until balancing is complete (meaning the dashboard is completely off). With an unbalanced battery pack, one balancing cycle takes approximately 20 hours. You can calculate the number of cycles needed based on the amount of charge missing from your own battery pack.
This leads to another question: After the equilibrium is completed, if you use a little electricity and then fully charge the vehicle, it will enter the equilibrium state again. Should this time be counted as an effective equilibrium? Based on the original poster's relevant experience, this equilibrium is almost ineffective.
Because the battery packs of the Tang and Qin are unbalanced, with one or two cells having excessively low voltage, a large number of other cells need to be discharged. While the system can correctly identify the remaining cells when the battery is low, it only identifies the cell with the highest voltage when fully charged at high capacity. This results in negligible efficiency.
Next, I'll discuss which batteries are problem-free and which are faulty. Here, I'm using the DCT software battery monitoring module from the 2014 Qin model to demonstrate the data. The Tang doesn't support this, but the battery pack principle is the same.
Many people find that their lowest voltage cell is only 2.6-2.8V when they have their batteries checked, and they suspect there's a problem with the cell, so they ask the dealership to replace it. The dealership then uses the manufacturer's form to give a normal answer, which makes the customer feel like the manufacturer is just giving a perfunctory response. In fact, it's normal for a single cell voltage to be low.
Ideally, at 5% charge, all battery cells should have a voltage below 3V, allowing the entire battery pack to release its full capacity. However, such a battery pack is practically nonexistent, requiring extremely high consistency across all cells. Generally, a good indicator of battery pack condition is that at 5% charge, the lowest voltage cell should be below 3V, and the highest voltage cell below 3.15V (this is the instantaneous voltage at 5% discharge; the voltage will recover after a short time, so there's no need to wait for it to recover).
Battery manufacturers have their own standards for replacement. If the conditions for replacement are met, you can choose to replace it. However, I recommend first using the correct equalization method to equalize the cells for 100 hours. If the effect is not obvious, then replace it. This is because it is difficult to match the replaced cells with the original cells that have already degraded.
If a battery cell has a problem and its actual capacity is reduced, then no matter how hard the balancing system tries, it will be ineffective. So how do we determine if a battery cell is faulty?
The voltage inconsistency caused by the battery leveling issue means that the lowest voltage cell at 5% and the lowest voltage cell at 100% are the same. However, a cell problem can cause the lowest voltage cell at 5% to have a higher or even highest voltage at 100%. If your battery pack exhibits this issue, there's no other way but to replace the problematic cell!
Insufficient charging capacity or insufficient pure electric range: There is a problem with the battery pack balancing or a problem with a certain battery cell. The solution is to first determine which situation it is, and the corresponding handling opinions have been introduced in the previous text.
Charging interruption: This occurs when the battery pack, while charging, reaches a certain percentage (e.g., 96%) and then skips the subsequent percentage steps, directly jumping to 100%. This happens because the system's displayed battery capacity is higher than the actual capacity. At this specific percentage, some cells have already reached the voltage required to terminate charging. Therefore, the system stops charging and simultaneously recognizes the battery level as 100%. The cause of this problem is insufficient charging.
When the battery level is low, the battery capacity drops rapidly: due to the discharge characteristics of lithium iron phosphate batteries, there is a long plateau in the middle where the voltage change is very low, and the system can only estimate the remaining capacity. However, when the remaining capacity of the cell reaches 15% (at which point the cell voltage is approximately 3.18V), the voltage will suddenly drop.
The battery management system of Tang and Qin will re-estimate the remaining battery capacity when a cell reaches this voltage. If the remaining capacity is displayed as 30% at this time, but the system re-estimates it to be only 15%, then the management system will increase the rate at which the displayed capacity decreases. As a result, where 1% could previously run 800 meters, it can now only run 400 meters.
The Dilemma of Domestic Battery Management Systems (BMS)
The development of new energy vehicles has not been smooth sailing. In the past two years, with the widespread adoption of new energy vehicles, we have also heard many "scandals" about them: spontaneous combustion, false driving range claims, etc. Why do these problems occur? The main reason is the lack of a battery management system or the use of an inferior or immature battery management system. In fact, the safety of new energy vehicles has always been one of the key priorities for the government and the automotive industry.
Not long ago, the Ministry of Science and Technology, the Ministry of Finance, the Ministry of Industry and Information Technology, and the National Development and Reform Commission jointly issued a "Safety Order" for the demonstration and promotion of new energy vehicles (i.e., the "Letter on Strengthening the Safety Management of the Demonstration and Promotion of Energy-Saving and New Energy Vehicles"). The order emphasizes that "all plug-in hybrid electric vehicles and pure electric vehicles put into demonstration operation must be equipped with a real-time vehicle operation technical status monitoring system (BMS), especially strengthening the monitoring of power lithium batteries and fuel cell lithium batteries." The causes of spontaneous combustion in electric vehicles are diverse, and simply installing a battery management system does not guarantee safety. For example, in terms of safety, accuracy, lifespan, and discharge capacity, a single battery cell can be charged and discharged 2000 times, but a battery pack may only be able to withstand 1000 cycles. If an immature BMS is installed, it cannot accurately monitor the battery charging and discharging status in real time, which can easily lead to excessive local power consumption and localized heat generation in the battery cells. Furthermore, the information cannot be transmitted to the driver, which can easily cause spontaneous combustion of the battery.
Industry experts believe that installing a high-quality Battery Management System (BMS) can effectively improve battery utilization, prevent overcharging and over-discharging, extend battery life, monitor the operating status of the battery pack and individual cells, effectively prevent battery pack spontaneous combustion, and provide early warnings to drivers in case of emergencies, thus gaining time to ensure safety.
The Future of New Energy Vehicles and Battery Management Systems
my country's new energy vehicle industry began in the early 21st century and has only been developing for a little over a decade. Due to people's desire for environmental protection and renewable energy, new energy vehicles have ushered in a development opportunity and have since grown rapidly. For a long time to come, new energy vehicles will continue to challenge the vast market that originally belonged to traditional fuel vehicles. Moreover, given the needs of social development, this encroachment on market share is to be expected.
While looking forward to the rapid development of new energy vehicles, we must clearly recognize that technological development is the foundation of industry development, and stable, efficient, safe, and reliable products are the embodiment of technology. We must understand that the current new energy vehicle industry in China is not friendly. Frequent electric vehicle spontaneous combustion incidents and false driving ranges 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, as well as the absence of authoritative organizations to conduct authoritative testing on BMS products manufactured by manufacturers, is the current predicament of the domestic BMS market. This has resulted in uneven quality of BMS products and made it difficult to promote them on a large scale.
Meanwhile, many domestic car manufacturers and battery pack companies currently lack sufficient understanding of the importance of BMS. They believe that as long as the individual battery cells can be connected, the vehicle can be guaranteed to operate. They take chances regarding its safety and blindly pursue low prices in BMS procurement. In order to secure contracts, some unscrupulous BMS suppliers have to reduce the functional specifications of BMS or simply remove some functions, thereby creating potential safety hazards. This is irresponsible and harmful to the entire industry.
Only by establishing unified industry standards as soon as possible, cracking down on manufacturers that do not meet market requirements, and establishing a sound testing system can battery management systems and new energy vehicles have a sustainable future. This is also the demand of many manufacturers and consumers.
