What is BMS?
We must first understand a definition, what is BMS?

BMS is actually the abbreviation of BATTERY MANAGEMENT SYSTEM. The Chinese name is Battery Management System. As the name suggests, it is a module dedicated to lithium battery operation and management. The object is lithium battery.

For general end users, the lithium battery protection board does not actually exist, or that they do not know that there is such a thing in the products they are using. For example, for electric vehicles, 100% of users know that there are batteries on electric vehicles because the batteries provide energy, but I can guarantee that at most 1% of users know that there is a lithium battery protection board.

The reason why BMS has such a low sense of existence is entirely because it cannot directly communicate with users, nor can it interact frequently with users. Even if some data is generated occasionally, these data are also transmitted through some dashboards. For users to observe, when they see the red light on the dashboard, they only say: "Well, the car seems to be broken, the quality is really bad."

Having said that, although BMS has a low sense of existence, its meaning is no less than that of the instrument, and it can even be said to be more important than the instrument, because it can detect whether the energy system of the car is broken, and only if you have the BMS With the system, the user may know whether the car is good or bad without taking risks.

If there is an embedded engineer in the industry who wants to buy an electric car and choose between an electric car without a display instrument and a BMS board, then he will definitely not dare to choose the latter, because if the electric car does not have an instrument, then the user The experience will be extremely poor, but if the electric car does not have a BMS... it is not so much an electric car as it is a bomb that may be activated at any time.

So why is BMS so important in the energy sector? What is the significance of the existence of BMS?

-------------------------------------------------- -------------------------------------------------- ---------

From the perspective of a low-level engineer, this article uses the BMS module for electric vehicles as an example to specifically discuss the design of the lithium battery protection board, and will give a reference plan. Of course, due to the author's limited ability and average level, if it appears in the article If there is an error or omission, please point it out directly.

What are the general requirements of BMS?
When the design team gets a project and starts development, it must first understand the requirements of the project. This requirement is not just a few words from the boss, but a practical, detailed and standard Documents. Documents should be structured as 1, 2, and 3, and clearly list all the requirements.

We now have a project: BMS board for low-speed electric vehicles.

As long as the project is successfully established, as a product manager or team leader, listing requirements documents is a must.

In the lithium battery management system, the key word is "management". To realize the management function, two modules are necessary. First: monitoring. Second: control. Only if I can monitor this system, I can control this system.

From this as the core point, I can slowly list the general requirements:

Monitoring the energy system can read all the status of the lithium battery in real time.
Control the energy system, and intervene when needed according to the status of the lithium battery (such as opening/closing the MOS tube of the charge and discharge loop).
 

How to realize the protection function?
data collection

The general requirements of the battery system are divided into acquisition and control, so what data will be collected?

Voltage
       Current
temperature
As a system to manage lithium batteries, the above 3 points are the data we need to monitor. If the system is super simple and only 3 or 4 points are collected, then the AD interface of the MCU can be used to read the information directly. But in reality, it is almost impossible to have such a situation, and the usual acquisition method is generally to use an analog front end.

system message
Voltage: The chemical characteristics of the lithium battery itself determine that we must protect the voltage. The so-called voltage protection means that we must ensure that the voltage of the lithium battery is always in the appropriate range. The voltage cannot be too low because of its internal storage. Electricity is realized by a reversible chemical change in electrochemistry. Excessive discharge will cause irreversible reactions to this chemical change. Therefore, lithium batteries are most afraid of overdischarge. Once the discharge voltage is lower than 2.7V, the battery may become permanent. Sexual damage, which is scrap. At the same time, the voltage should not be allowed to be too high, because once the battery is overcharged, the damage caused is far greater than overdischarge. Overdischarge can damage the battery at most and will not cause harm to the surroundings. Overcharge may cause the battery temperature to rise to the point Spontaneous combustion or even explosion, this hazard is fatal.

Current: The so-called current protection is also we must ensure that the current cannot be too large whether it is charging or discharging. Everyone knows that a short circuit is a manifestation of overcurrent. When the positive and negative poles of the system are in direct contact, the resistance of the wire Small, resulting in a very large current, and a very large current will generate a lot of heat, and the resulting combustion and explosion are very fatal. In fact, even if it is not a short circuit, the excessive current will still cause the internal heating of the battery, which is very likely to cause permanent damage.

Temperature: Needless to say, temperature protection. Except for the chemical characteristics of the lithium battery itself that make it impossible to use at extreme temperatures, I think any similar system should take temperature into consideration for safety reasons.

When you see this, everyone should understand that the logic of the lithium battery protection system is actually very simple. It is nothing more than some judgments and the control of MOS tubes. These things should be able to be realized by college students who have a software foundation.