lithium 3400mah 3.7v 18650 battery.What is the way to innovate BMS products for electric vehicles?

　　To borrow a term from the economic circle, electric vehicle BMS products are currently in a state of "stagflation." Although product functions are constantly improving and market applications are expanding, the key technical levels of the products are still stagnant, and the original pain points Still exists. One of the main reasons for this situation is that our product design solutions all use special battery management ICs provided by foreign semiconductor IC manufacturers, and design based on their application solutions. BMS products can be divided into software Algorithms and hardware architecture are two relatively independent parts. Because software algorithms are more professional and sophisticated and have little connection with the hardware part, this article only analyzes and discusses the connection between hardware architecture and product functional performance.

　　Let’s start with the battery management dedicated chip. The emergence and development of battery management-specific ICs are closely related to various problems encountered in the application of lithium batteries. The charge and discharge protection chip for a single battery was first designed to solve the problem of overcharge and overdischarge of lithium batteries. Later, in the application of multi-cell lithium batteries in series, chips for multiple strings were developed. At this time, it became the battery management chip. The chip mainly collects the voltage data of each battery in the battery pack. In the future, in order to deal with the problem of battery inconsistency, the driving function of the power switch will be further integrated. This is the battery management IC with balancing function.

　　Objectively speaking, battery management-specific ICs enabled the early BMS industry and led the development of BMS products. It is precisely because of the dedicated chip that the design of BMS can be greatly simplified, and the miniaturization and reliability of the product have been greatly improved. However, at the same time, we must also see the limitations of the dedicated chip. As mentioned earlier, battery management dedicated chips also developed with the application of lithium batteries. Early lithium batteries were mostly used in small electronic devices, and later became widely used in notebook computers. So far, battery management dedicated chips have been used for low-series batteries. Number and small equipment services. When lithium battery packs are applied to electric vehicles, the situation begins to change. Lithium battery packs for electric vehicles are high-series, large-capacity batteries used in series. The number of dozens or even hundreds of series is no longer comparable to the single-digit series use of several series in laptops. Special-purpose ICs have not been idle, and have rapidly launched products with more string applications, but considering the voltage and application complexity, they generally do not exceed 20 strings. The typical architecture of a BMS designed using these ICs is a centralized architecture. There are only connections between the BMS and the battery pack. The number of connections depends on the number of battery strings. The number of dedicated battery management chips on the BMS circuit board also depends on the number of battery strings.

　　As can be seen from the schematic diagram, the advantages of centralized BMS products are simple structure and low cost. When the number of battery strings is low, such as 10 or so, the wiring is not too complicated. Moreover, when the battery string capacity is small, the BMS can be installed close to the entire battery string, shortening the wiring distance. The battery string - ---BMS, the entire energy system is relatively compact and integrated, and is more suitable for electric bicycles and electric motorcycles. However, when applied to lithium battery packs for electric vehicles, because the battery capacity is required to be large and the physical size of the group is relatively large, the wires will be longer and vary in length. In addition, there are many strings and the number of wires will be large. The arrangement of dozens or even hundreds of lines is very troublesome. Another important detail is that the order of these connections needs to be fixed, because the pins of the dedicated chip have pre-defined the battery series connection sequence, so the connections on each string of batteries must be connected to the connector pins designated by the BMS. . Although this is not difficult in the BMS design work, it is a big trouble in the actual connection work of the BMS and the battery pack. Generally, one end of the wire is connected to the battery, and the other end is connected to the BMS through a plug-in. The work of connecting to the battery is now done manually, and it will be difficult to do it by machine in the future. The wires connected to each electrode of each battery string are all connected. No mistakes can be made, the entire workload can be imagined. Through the analysis of centralized architecture, we see that dedicated ICs are more suitable for small-capacity, low-string-count situations. In large-capacity, high-string-count situations, there are disadvantages of complicated wiring and the need for one-to-one correspondence.

　　Looking at the issue of balancing again, a centralized architecture is more suitable for passive balancing without increasing the complexity of the circuit design. Nowadays, mainstream special-purpose ICs also have this function. However, the current capacity is limited, at the 100 mA level. When the consistency difference is not big in the early stage of use of the battery pack, the problem is not big. When the consistency difference is large in the middle and later stages, there is a risk that the battery imbalance cannot be corrected. . If you want to add active equalization function, the existing architecture is basically of no help. Additional wiring harnesses and switch matrices are required, and the circuit complexity increases sharply. The switch matrix requires a large number of electronic switches and MOSFETs. However, due to the large number, the control circuit is quite complex. Some companies use relays instead, which simplifies the design, but brings problems with the life of the relay as a mechanical switch and the risk of malfunction. Of course, you can also extend the life of the relay by reducing its switching frequency and avoid risks by checking for malfunctions. However, doing so cannot guarantee the mean trouble-free working time of the device, not to mention that the number of relays is quite large, more than one. This is a compromise of last resort rather than a solution to the active balanced switch matrix conundrum.

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