1. Introduction to BMS functions

The battery management system (BMS: Battery Management System) plays a decisive role in battery pack management. It constantly monitors the voltage, internal resistance, SOC changes of each battery, as well as the voltage, charge and discharge current, real-time SOC estimation, temperature and other important parameters of the entire battery pack. , when abnormality occurs in these parameters, information is displayed in a timely manner, early warning prompts are issued, and even protection actions such as shutdown are automatically performed.

2. BMS key technologies and shortcomings

Although the function of BMS in battery pack management is powerful, it is only superficial. The core function of BMS is battery management. Since it is called a management system, it should be managed in place and cannot stay in the monitoring of various states and parameters. on, allowing users to intervene, especially in terms of safe operation and battery life stability.

The existing BMS technology is far from meeting the expected requirements in the management of core issues, and it does not play its due management role. There are serious technical and management shortcomings. Here is a brief analysis:

First, BMS failed to solve the problem of battery pack consistency. The consistency problem is the most common problem that is most likely to occur after the battery pack has been running for a period of time. It is a common problem and a technical difficulty in battery management. Many problems are caused by it. It is a technical problem in battery management worldwide.

Battery management system

Take electric vehicle battery packs as an example. Although vehicle battery packs are strictly selected before assembly to ensure very good consistency, and are always under the monitoring and management of BMS, consistency problems may occur after a period of use. (The initial consistency problem is slight or even negligible), and as the use time increases, the consistency problem will gradually accumulate and worsen. The most intuitive manifestation is that the actual cruising range gradually shrinks, and the charging capacity also gradually decreases. This The degree of reduction in cruising range and charge capacity is usually higher than the value specified in the vehicle manufacturer's manual.

Second, BMS does not fully utilize the operating efficiency of the battery pack. After a serious inconsistency problem occurs in the battery pack, in order to prevent the "lagging" battery from overcharging and over-discharging, the BMS will issue high-voltage and low-voltage warning messages in a timely manner, and will promptly close the charging channel and discharging channel. This will cause the normal battery to be unable to be fully charged and discharged. Without effective discharge, the capacity cannot be effectively utilized. For electric vehicles, the cruising range will be severely reduced in a short period of time.

The original intention of developing BMS is to ensure the stable operation of the battery pack. Stability here not only includes safe charging and discharging and stable operation, but also includes continuous stability of capacity. Capacity stability mainly means that the attenuation of the capacity of single cells and the entire battery pack must comply with the normal attenuation law of the battery and within the designed cycle life of the battery pack. , the capacity of the battery pack should decay smoothly and reasonably. If the reasonable decay of the battery pack cannot be guaranteed, it means that the functions and functions of the BMS have not been realized.

Third, passive equilibrium is a no-brainer. The highlight of passive balancing is that it can ensure that all batteries of different capacities can be fully charged during charging, that is, balanced charging. This is what all manufacturers strongly promote. Regarding the inherent defects of passive balancing, such as small-capacity batteries that are prone to over-discharge, There is no mention of the slow balancing speed and reduced battery energy utilization. Passive balancing cannot solve the problem of balanced discharge.

We all know that the capacity of the battery pack depends on the discharge capacity, and the discharge capacity depends on the battery with the smallest capacity. The execution of passive balancing will make the battery with the smallest capacity always work at a relatively high charge and discharge rate. It decays relatively quickly, causing its capacity to become less and less, ultimately leading to a rapid decline in the battery pack's usable capacity. In addition, since the current of passive balancing is very small, passive balancing can only play an auxiliary role in preventing overcharging of small-capacity batteries.

Fourth, it affects SOC estimation. Real-time SOC estimation is one of the most important functions of BMS. The existence of consistency problems and attenuation problems will cause the voltage change of the attenuated battery to increase, directly affecting the accuracy and error of SOC estimation, and affecting the estimation of remaining cruising range and travel routes. and program decisions.

For example, the voltage of a decaying battery will drop rapidly during discharge, especially near the end of discharge, and the estimated SOC value will also drop rapidly instead of slowly. When the discharge is stopped or the discharge current is greatly reduced, the voltage of the decaying battery will It will rebound significantly, causing the SOC estimate to rebound quickly. This situation is also called SOC jump, which will seriously affect the accuracy and relative stability of the SOC estimate, and directly interfere with and affect users' travel decisions.

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