What is the core technology of the battery management system (BMS)?

　　BMS is the core component of lithium batteries, especially energy storage systems, electric vehicles and other products that need to be applied to large-scale battery combinations. Its importance is beyond doubt.

　　Many BMS manufacturers on the market claim to have their own so-called core technologies, and some new terms are constantly emerging, causing headaches for R&D engineers. So, what is the core technology of the real BMS?

　　A: We must be clear that the BMS system usually includes a detection module and a calculation control module.

　　Detection refers to measuring the voltage, current and temperature of the battery cell and the voltage of the battery pack, and then transmitting these signals to the computing module for processing and issuing instructions. So that the computing control module is the brain of BMS. The control module generally includes hardware, basic software, runtime environment (RTE) and application software. The core part-application software.

　　The function of the software is generally divided into two parts: the estimation algorithm of the battery state, the fault diagnosis and the protection.

　　State estimation includes SOC (State of Charge), SOP (State Of Power), SOH (State of Health), balance and thermal management. Battery state estimation is usually to estimate SOC, SOP and SOH. SOC simply means how much power is left in the battery; SOC is the most important parameter in BMS, because everything else is based on SOC, so its accuracy and error correction capabilities are extremely important. If there is no accurate SOC, no amount of protection function can make the BMS work normally, because the battery will always be in a protected state, and it will not be able to extend the life of the battery.

　　In addition, the estimation accuracy of SOC is also very important. The higher the accuracy, the higher the cruising range for the same capacity battery. Therefore, high-precision SOC estimation can effectively reduce the required battery cost. For example, the Fiat 500e BEV of Chrysler can discharge SOC=5% all the time. It became the electric car with the longest cruising range at that time.

　　The accurate estimation of SOP can maximize the utilization efficiency of the battery. For example, when braking, it can absorb as much energy as possible without harming the battery. When accelerating, it can provide more power to obtain greater acceleration without harming the battery. At the same time, it can also ensure that the car will not lose power due to undervoltage or overcurrent protection during driving, even when the SOC is very low.

　　In this way, the so-called primary protection and secondary protection are incidental products in the face of precise SOP. Not that protection is not important. Protection is always needed. But it cannot be the core technology of BMS.

　　For low temperature, old batteries and very low SOC, accurate SOP estimation is especially important. For example, for a group of well-balanced battery packs, when the SOC is relatively high, the SOC difference between each other may be very small, such as 1-2%. But when the SOC is very low, the voltage of a certain cell will drop rapidly. The voltage of this battery cell is even more than 1V lower than the voltage of other batteries. To ensure that the voltage of each cell is not lower than the minimum voltage given by the battery supplier, SOP must accurately estimate the maximum output power of the cell whose voltage drops rapidly at the next moment to limit the use of the battery and protect the battery.

　　The core of estimating SOP is to estimate each equivalent impedance of the battery online in real time.

　　SOH refers to the state of health of the battery. It includes two parts: changes in capacity and power. It is generally believed that when the capacity decreases by 20% or the output power decreases by 25%, the battery life is reached. However, this does not mean that the battery can no longer be used.

　　For a pure electric vehicle EV, capacity estimation is more important, because it is directly related to the cruising range, and the power limit is only important when the SOC is low. The requirement for SOH also requires both high precision and error correction capability. And SOH without error correction capability is meaningless. The accuracy is less than 20%, and it is meaningless.

　　The estimation of SOH is also based on the estimation of SOC. So that the SOC algorithm is the core of the algorithm.

　　The battery state estimation algorithm is the core of BMS. Everything else serves this algorithm. So that when someone claims to have broken through or mastered the core technology of BMS, one should ask him what exactly did BMS do? Is it an algorithm or an active balance or only the hardware and underlying software of the BMS? Or just propose a BMS structure?

　　Some people say that Tesla is great because its BMS can manage 7,104 batteries. Is this its power? Does it really manage 7104 batteries?

　　Tesla Model S does use 7104 batteries, but only 96 batteries are connected in series, and the parallel connection can only be counted as one battery, no matter how many batteries you connect in parallel. Why? Because the battery packs of other companies only count the number of batteries in series instead of the number in parallel.

　　Why is Tesla special? In fact, if you understand Tesla’s algorithm, you will know that Tesla’s algorithm not only requires a large amount of working condition data to calibrate, but also cannot guarantee that in any case, especially Estimated accuracy after battery aging.

　　Of course, Tesla's algorithm is already at the top of the century. Ordinary BMS algorithms almost always use current integration plus open circuit voltage to calculate the initial SOC using open circuit voltage, and then use current integration to calculate the change in SOC. The problem is that if the voltage at the starting point is wrong, or the capacity is incorrect, don’t you have to make a mistake to the end, and it will not be corrected until it is fully charged again?

　　Will the voltage at the starting point go wrong? Experience tells us that it will, although the probability is very low. If you want to be foolproof, you can't just rely on the accurate voltage of the starting point to ensure that the starting SOC is correct.

　　B: What kind of algorithm is a good algorithm?

　　From a control point of view, a good algorithm should have two criteria: accuracy and error correction capability. The truth is that the higher the accuracy, the better is the most obvious. Really doing well in the algorithm is using online real-time estimation of open circuit voltage to achieve online real-time error correction.

　　Why is the real-time online estimation emphasized here? What are its benefits? All the equivalent parameters of the battery are estimated through real-time online estimation, so as to accurately estimate the state of the battery pack. Real-time online estimation greatly simplifies the calibration of the battery. This makes it a reality to accurately control the state of the battery pack with poor consistency. Real-time online estimation enables both new batteries and aging batteries to maintain accuracy and robustness or errorcorrection capability.

　　Some people often don't know what other people's algorithms are. When a certain manufacturer produces certain parts of a BMS for a certain manufacturer, they think that they have mastered the core technology of BMS. Such a statement is not correct.

　　C: What are the characteristics of the best BMS in the world?

　　It can estimate the battery parameters of the battery pack online in real time to accurately estimate the SOC, SOP, SOH of the battery pack, and can correct the error or percentage of the initial SOC exceeding 10% and the error or percentage of the ampere-hour capacity exceeding 20% in a short time A few of the current measurement error.

　　When General Motors Corporation of the United States developed Volanda 6 years ago, it did an experiment to test the error correction ability of the algorithm: remove one of the three battery packs connected in parallel, and at this time the internal resistance increases by 1/3, When the capacity is reduced by 1/3. But BMS does not know. As a result, SOC and SOP were all corrected in less than 1 minute and SOH was then accurately estimated. This not only shows the powerful error correction capability of the algorithm, but also shows that the algorithm can always maintain the same estimation accuracy throughout the battery life cycle.

　　For computers, if a blue screen appears, we generally only need to restart the computer. However, for a car, even if the probability of breaking down is only one in ten thousand, it is intolerable. Therefore, unlike publishing articles, automotive electronics needs to ensure that it can work under any circumstances.

　　To make a good algorithm requires a lot of energy to solve those situations where the probability of occurrence is only one in a thousand or one in ten thousand. Only in this way can it be guaranteed to be foolproof. The precise mathematical model is the diffusion equation described in the textbook. But it cannot be used in the car because the computational complexity of the numerical solution is too large. The CPU computing power of the BMS is not enough. This is not only an engineering problem, but also a math and physics problem. Solving such technical problems can resolve almost all known polarization problems that affect battery state estimation.

　　D: The state estimation technology of BMS is the core technology of BMS.

　　Although 6 years have passed, there is still no supplier in the world that can achieve such high accuracy and high error correction ability to ensure the foolproof operation of the battery.

　　Even Tesla is unable to achieve perfect estimation technology. Because Tesla's algorithm cannot guarantee the accuracy and error correction ability of the battery after aging. Otherwise, why would there be so many Tesla cars in need of emergency rescue?

　　Only algorithms that can guarantee high accuracy and high error correction capabilities are the core technology of BMS!

　　Lithium-ion battery (LIB) has become the main energy storage solution in modern social life. Among them, lithium iron phosphate batteries are a perfect replacement for lead-acid batteries, and they are the first choice for grid-connected peak shaving, off-grid energy storage, photovoltaic energy storage, UPS, data center and other industries.