Classification and function of BMS

1. What is BMS

1.1 BMS definition

Well, we finally got to the point, first of all, what is BMS.
BMS is the abbreviation of BATTERY MANAGEMENT SYSTEM.
Basically, a power supply system composed of more than two single cells requires a BMS.

After discussing the battery in our last article, I think we all already know a BMS Basic functions that should be available.

1.2 Basic functions of BMS

• Battery monitoring
  first is battery monitoring. BMS needs to monitor various states of the battery, including voltage, current, temperature, etc. Generally speaking, both the single and the total need to be detected. According to the complexity of BMS, there are different structures. Will talk about it.
• Battery protection After
  we get the data, we then provide the necessary protection for the battery based on the data, and let the battery work in a safe environment, which mainly includes voltage and current control and temperature management during charging and discharging.
• The battery state estimation is
  then an estimate of the state of the battery based on previous data, such as SOC. This data cannot be measured directly. It needs to be measured by other methods, such as the use of voltage changes to characterize the capacity, but this This type is often inaccurate, because the voltage curve of the battery charge and discharge is almost flat within a certain range, which leads to a large error; a more complicated one is to use the integration of the voltage and current according to the charge and discharge time. , It will be a little better than before; then there are some Kalman filter algorithm to achieve, this is a good algorithm that can be used to predict, it also takes into account all kinds of noise; there are now more popular nerves The Internet is used, and AI is used for management, but I do n’t know if there are any mature products out.
• To maximize battery performance,
  our BMS also needs to specify different balancing strategies through analysis of all data. The ultimate goal is to charge it as full as possible and discharge it as much as possible when discharging, and do not exist as we have shown before. Extremely uneven phenomenon.
• Feedback to users or external devices
  Finally, what is needed is to communicate with the outside world, generally speaking to communicate with our other ECUs via CAN bus.

Generally speaking, the BMS actually solves two things. The first thing is safety, ensuring that the battery has a good working environment, and ensuring that the rain and dew are all covered, so that individual batteries are not always suffering and causing dissatisfaction. The main reason is that the result is terrible. Either strike or explode; followed by high efficiency, let each single cell play its own maximum value and produce the highest benefit. ,

2. Classification of BMS

Next, let me introduce the classification of BMS. When it comes to classification, the first thing to mention is the classification criteria, which is to classify according to what. Here I will introduce you from two aspects.

2.1 Classification by function

The first is the classification by function. According to the function, we will go from simple to complex one by one. The
first is the simplest, called

• Constant current and constant voltage charging, this can't even be called a battery management system. Some simple chargers are this strategy, because it is to provide a constant current or voltage, which is used in battery cars that use lead-acid batteries in many areas. Equipment, but because lead-acid batteries are relatively solid, they are generally not dangerous. However, we know from the previous introduction that this simply cannot provide any protection for the battery, but without other control strategies, then this is equivalent to directly exposing the battery to an unprotected environment, which is more dangerous.

• The second type is called a shunt, which is also relatively simple. The simplest form is to connect a Zener diode in parallel with the battery to form a voltage clamping circuit, which is basically equivalent to simply monitoring the voltage. When the voltage reaches the set value At the time, the diode breaks down in the reverse direction, diverting most of the current to protect the battery. But there are also some problems. The rated current of the shunt is limited. When the current is greater than the upper limit of the shunt, there will still be current flowing through the battery. So either the shunt is large enough, or the BMS of the shunt needs to be connected with the charger Matching, the charger can dynamically adjust the voltage in conjunction with the action of the shunt to achieve better results. And it seems that there are relatively few shunts for each battery. The shunt may only reach the level of the battery pack.
  

• A third detector is, a so-called detector, is provided with only the measurement functions, and can be measured overall cell voltage, current, temperature, etc. SOC estimates, they will be presented on the screen or elsewhere,
  
  which It is a schematic diagram of a monitor. As you can see, in fact, it cannot be used as a complete BMS. Why? Although the data can be displayed, it cannot prevent overcharging and overdischarging, nor can it achieve balance, but why should I list it?
  Because in some cases, the scene where we use the battery is someone involved, as long as we add people to this system, it is complete, and we can perform corresponding actions according to the parameters.

• After talking about the monitor, then we came to the monitor, what's the difference? In fact, the control action is added to the monitor, so that a closed loop is formed, which can automatically work and perform the corresponding action according to the monitored data, such as using the action of the relay to cut off the charger or the load. The battery can be protected from being overcharged and discharged, thus ensuring the safety of the battery. But its shortcoming is that it can not balance the power between the single cells.
  

• Next we continue to enrich the function, equalizer.
  
  The equalizer adds a single cell balancing function on the basis of the monitor, which can achieve active and passive balancing, thereby ensuring the optimal use of the battery.

2.2 Classification according to topology

Topology is very important for BMS, because it will affect the cost of the system, reliability, ease of installation and maintenance, and accuracy of measurement and prediction.

2.1.1 Centralized

The first is the first type, the centralized type, also called the integrated type. The
centralized type is to encapsulate the entire BMS in a device, throw out the wire and connect it to the conductor battery.
As shown in the picture,

his advantages are very obvious,

• The first is the compact structure, often a box
• Then the price is cheaper
• Also, the maintenance is relatively simple, and it can often be replaced as a whole.

But there will be deficiencies,

• The first is poor scalability, a product has to be redesigned after it is finalized,
• Secondly, there are hidden dangers, because if the harness is too long, it will cause a series of hidden dangers.

2.1.2 Modular

The next one is the modular type

. It is very similar to the centralized BMS, but the modular BMS is divided into many same sub-modules. Does each packaged wire connect to different parts of the entire panel. To monitor a certain area,
we It is said that the modules have the same function, but in fact there will be a module assigned as the main module, which is used to manage and schedule the entire battery pack and is responsible for communicating with the outside world. The other slave BMSs communicate with the master BMS through the communication bus, but the functions between them are the same.
Its advantages are:

• First of all, because it is equivalent to miniaturizing the centralized BMS and multiple cascades, it has most of the advantages of centralized, such as convenient maintenance, cheap prices, etc.
• Secondly, due to the small size of a single module, the wires from the sub-module to the single battery will be relatively short and can be closer to the battery, thus avoiding the hidden dangers and errors caused by excessively long wires.
• Finally, it is also easy to expand, add more sub-modules to achieve expansion

But it also has some disadvantages,

• The first is the need to add additional wires. Compared to the centralized type, the modular step needs to be connected to the battery pack, and wires are also required between each module.
• Secondly, the cost is higher, the main reason is that the function of each module is the same, but not all functions are used, which causes waste, especially the slave module, its practical function is not much

In fact, we see here that this structure is not particularly reasonable, so there is an improved version.

2.1.3 Master-Slave

In fact, we see here that this structure is not particularly reasonable, so there is an improved version. We separate the modules according to the different functions of the master and slave

The functions that are not used by the module will be removed, so that a large amount of cost can be reduced
. The functions that the master BMS is responsible for are relatively large, including calculation, prediction, decision-making, communication, etc. The slave unit is basically only responsible for measurement.
In this way, it can be said to inherit most of the advantages of the modular structure, while also reducing the cost of expansion.

2.1.4 Distributed

There is also a topology, which is slightly different from what we talked about before, called distributed

In the previous topologies, various electronic devices are not installed on the single battery, and are basically measured by throwing the wire in the past.
However, in a distributed system, our measurement unit and other electronic equipment are directly installed on the circuit board integrated with the single battery. This advantage is that compared with the previous ones, the connection between the BMS and the single battery Basically wiped out.
Then it is a bit like the master-slave type, it will also have a controller to be responsible for calculation, prediction, decision-making and so on. Modules communicate based on the bus. In cars, we generally use the CAN bus.
Each of our units contains an acquisition loop. MCU with CAN bus can directly send and receive information through bus communication.

Its advantages are naturally many:

• The first is to have extremely high scalability, which can be fine to the expansion of single cells.
• Secondly, the connection reliability is high. Basically, there are no cables that are too long. The battery and the measurement circuit are closely integrated, which also reduces interference and errors. Security is also very high.
• At the same time, it is also easy to maintain. If something breaks, only a small unit needs to be replaced.

But there will be some shortcomings:

• For example, first of all, the cost is very high, because each monomer has a set of equipment, so the overall cost is very high.
• Secondly, the volume is too large. This is also easy to understand. There is a measurement system next to each cell of each battery, which will affect the volume of the entire panel.

Now some ICs can also be made very small, this influencing factor will become increasingly weak.

3. Functions of BMS

3.1 Basic functions of BMS

Speaking of the function of BMS, let ’s first infer based on the previous content.

First of all, we have a pack of batteries, lithium-ion batteries, which are very powerful, but have a short-tempered temper. How to design it to manage it?

• What is the first function we need, definitely measurement, we need to measure all the information we want to know from the battery, after getting the information?
• We certainly will not make a simple display. We need to analyze the data. When we have a result based on the data analysis, what should we do?
• First of all, it must be control, control the charging and discharging process, control temperature, etc.
• Secondly? We need to inform the results of the analysis to other controllers, similar to vehicle control, dashboard, etc.
• In addition to communication, our results should also be saved, because there will be some faults in this process, and fault codes will be generated, also for future traceability when we analyze the problem, the log function is necessary, and the log also needs to pass The communication module communicates with the outside world, and this process is bidirectional
• At the same time, the commands we received from the outside also need to be added to the analysis process, so this process should also be bidirectional.

This is the basic function of a BMS.

3.2 Typical BMS functions

Let ’s take a look at what features a real generic BMS will contain

• The first one is the battery parameter test, which is mainly for the single battery, including the voltage, temperature, and internal resistance of the battery. The
  voltage collection is basically now collected with a special ADic, but if If you look down, you will actually have a different structure. Some are equipped with an ad channel for each single cell. We call it discrete acquisition. Some use a positive electrode to scan. We call it unipolar multiplexing. The multiplexing to scan, we call differential multiplexing, of course, this is mainly determined by the cost of the chip.
  The temperature detection is generally done with a thermistor, but the difference is the amount and placement of the thermistor. If each monomer is monitored, it is naturally the most accurate, but the cost is bound to rise, and More wiring harnesses will be added, so there is another way to monitor by region, by dividing the battery pack into different areas to monitor the temperature, the corresponding cost will be reduced, and if the monitoring point is more reasonable, similar effect. This problem has a natural advantage for distributed BMS. Distributed BMS can integrate temperature measurement function relatively easily.
  For current, there is no way to measure it directly, usually with a shunt or Hall sensor.
• The second is the measurement of the system parameters. We just talked about the unit. This is a control of the whole. Including the measurement of total voltage, total current, and insulation resistance, this insulation resistance is also mandatory to ensure safety in national standards.
• The third function is related to the calculation of energy, which is to use various methods to calculate the parameters of SOC, SOH, DOD and other indicators that can measure the battery.
• The fourth is battery management, including the distribution of battery energy, battery sleep and wake up
• The fifth is thermal management, because the normal operating temperature range of the battery is relatively narrow, so the temperature of the battery needs to be managed. To put it bluntly, it is cold and heated, and if it is too hot, think of ways to cool down, blow or liquid cool.
• The sixth and seventh functions we talk about together are the control of the power and the way of charging and discharging.
• Plus the eighth balance management, including active balance and passive balance, can basically ensure that each single cell can be fully utilized.
• In addition, it is necessary to provide the necessary protection for the entire system, so the relay control is indispensable.
• The next step is to diagnose the fault, including classification, classification and treatment of some faults
• The next step is to process the data, including log records, fault code records, and some application data management.
• The last one is communication, which is generally achieved through the CAN bus.

We have almost summarized the 12 major functions, then we classified these functions, and we found that the functions we thought before were almost the same.

In the next section, we talk about BMS testing, so stay tuned.

Reference materials: http://www.hongcesys.com/