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The NTC in temperature sampling was introduced earlier. In fact, there is nothing about the design of temperature sampling circuit. I am ready to write it again; In the second part, I want to change the topic, otherwise it will be boring. I want to introduce the whole picture of BMS, first the whole, then the part, first the whole, then the function. I think there will be many new thoughts and gains in the process of summary and decomposition.

I won't be verbose about the basic functions and functions of BMS. There are a lot of online searches. It's not necessarily that I write more comprehensive than others; BMS is probably like a babysitter for lithium-ion batteries. Without it, lithium-ion batteries will be out of control once they get into trouble; So I always think that BMS is just a temporary worker. When the lithium-ion battery is mature, it will not need BMS, and I will lose my job. Well, after losing my job, I will write a novel, and I don't know if anyone will read it.

First, correct the definition of BMS, which is described as follows in the national standard QC/T897-2011:

BMS defined in the standard includes controller and collector, which is an electronic component; The controller is called BCU, and the collector is called BE. Although the name of the latter is rather blunt, it is orthodox.

However, the names in reality are different. The names of controllers include BCU, BMU, BMC, BECU, etc. The names of collectors include BMU, BIC, CMU, CMC, etc; It's not important to call it, just unify the language.

Then it is still unavoidable to look at the position of BMS in the battery pack, as shown in the following diagram:

The BMS is generally placed inside the battery pack, and the controller is also placed outside; Speaking of this, it is now popular to package and combine the three in one, four in one, OBCDCMCUVCU and other combinations, as well as propose to combine the controller in BMS with VCU.

As for the controller alone, the basic functional requirements and implementation scheme have not changed much compared with the previous ones, but there is an additional requirement for functional safety. Functional safety is a system engineering. Finally, when it comes to controller hardware, it requires us to select chips with functional safety certification when selecting complex chips (such as single chip computers, power chips, etc.). Moreover, the concept of functional safety should be integrated into the scheme design at the beginning, which puts forward higher requirements for hardware engineers. It needs to be considered from the perspective of the system. It is a good thing for hardware engineers to further master the operation scenario of the whole vehicle. Functional safety mainly solves the problem of safety, but it is not the same as the reliability of the product. It is not likely to improve the reliability of the product, or even reduce the reliability of the product; And the compromise of cost is also a difficult problem.

The collectors must be placed inside the battery pack, but they are all internal. Some are integrated inside the module, some are placed on the module, and there is a centralized scheme to connect with the battery through a long harness.

The technical scheme of the collector was first to use discrete components to overlap the sampling circuit (although there are still a few), and then it was gradually replaced by the current integrated AFE; There are many kinds of topologies, distributed, centralized (see many centralized requirements recently), or a combination of the two; Communication topology also includes CAN and daisy chain schemes.

AFE, the key chip in the collector, is basically monopolized by large foreign manufacturers, which started relatively late in China; In particular, American semiconductor manufacturers: Meixin, ADI, TI, and other countries, as well as Panasonic, ST, NXP, etc., have a lot to say in this, and I will write slowly later.

BMS design has been criticized by others because it can not prove its competitiveness with several key indicators like power circuit; For BMS, others will say that sampling is implemented with AFE, which is done well by semiconductor manufacturers, and has little to do with hardware designers; The accuracy of SOX calculation is too difficult to prove, resulting in an embarrassing situation: hardware is difficult to prove excellent, software is difficult to prove excellent, which makes people feel that there is no technical content.

This is because the entry threshold of BMS is low. Like the guitar in musical instruments, it can play 53231323 after one class. It seems very powerful, but it is not good to start; It took many people an hour to understand the BMS implementation plan, but it is still far from enough. It is also possible that BMS basically belongs to the category of digital circuit and lacks the mystery of power circuit.

Regardless of the scope of knowledge involved in BMS, the number of signals processed by BMS is enough, and the processing object is the most complex cell, and different types of cell should be adapted. The signals processed by BMS are also rich enough: battery core, collision, CAN, charging, water pump, high voltage, insulation, etc., each of which will involve a series of knowledge points.

Note: The picture is from book.liionbms

summary

To do a good job of BMS, you need to know too much. The most basic device production process, device selection and device failure mode need us to spend a lot of time to learn, and there are many types of devices involved; Then there are various standards, including test standards, design standards, etc; Then came the specific circuit design, derating design, PCB design, etc; Then the verification and rectification of functions and performance; BMS does not omit any steps because it looks simple. On the contrary, because BMS is closest to the battery, it is often impacted more strongly, which brings many unique aspects in design.

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