About Lithium Battery Pack Monitoring, SOC and Battery Balancing Technology

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About Lithium Battery Pack Monitoring, SOC and Battery Balancing Technology [Copy link]

The BMS electric vehicle battery management system is an important link between the on-board power battery and the electric vehicle. BMS collects, processes and stores important information during the operation of the battery pack in real time, exchanges information with external devices such as the vehicle controller, and solves key issues such as safety, availability, ease of use, and service life in the lithium battery system. Its main function is to improve the utilization rate of the battery, prevent the battery from overcharging and over-discharging, extend the battery life, and monitor the battery status. In layman's terms, it is a system for managing, controlling, and using battery packs. The three core functions of BMS are battery cell monitoring, state of charge (SOC) estimation, and single cell balancing. 1 Battery monitoring technology 1. Single cell voltage acquisition; 2. Single cell temperature acquisition; 3. Battery pack current detection. Accurate temperature measurement is also very important for the working state of the battery pack, including temperature measurement of a single battery and monitoring of the heat dissipation liquid temperature of the battery pack. This requires the reasonable setting of the location and number of temperature sensors used to form a good match with the BMS control module. The focus of monitoring the heat dissipation liquid temperature of the battery pack is the fluid temperature at the inlet and outlet, and the selection of its monitoring accuracy is similar to that of the single cell. 2 SOC technology Single cell SOC calculation is the focus and difficulty in BMS. SOC is the most important parameter in BMS, because everything else is based on SOC, so its accuracy and robustness (also called error correction capability) are extremely important. Without accurate SOC, no matter how many protection functions are added, the BMS cannot work properly, because the battery will often be in a protected state, and the battery life cannot be extended. The higher the accuracy of SOC estimation, the longer the driving range of electric vehicles for batteries of the same capacity. High-precision SOC estimation can maximize the performance of battery packs. The most commonly used calculation methods are the ampere-hour integration method and the open-circuit voltage calibration method. By establishing a battery model and collecting a large amount of data, the actual data is compared with the calculated data. This is also the technical secret of each company, which requires a long time and a large amount of data accumulation. It is also the part with the highest technical content of Tesla. Tesla has applied for more than 100 core patents in the fields related to BMS such as battery cooling, safety, and charge balance. 3 Balancing technology Passive balancing generally uses resistance heat release to release the "extra power" of high-capacity batteries to achieve the purpose of balancing. The circuit is simple and reliable, and the cost is low, but the battery efficiency is also low. Active balancing transfers excess power to high-capacity cells during charging and to low-capacity cells during discharging, which can improve the use efficiency, but the cost is higher, the circuit is complex, and the reliability is low. In the future, as the consistency of battery cells improves, the demand for passive balancing may decrease.

Almost all mainstream automotive BMS manufacturers have passive balancing technology, and most of them have active balancing technology reserves. The installed capacity of passive balancing BMS is large, occupying a higher share of the new energy vehicle market, far higher than the market share of active balancing BMS. The fundamental reason is the cost factor. Active balancing is more of an "optional" function.

Taking into account the consumption habits of the Chinese market, the current domestic new energy vehicles are mainly mid- and low-end brands. In order to strictly control costs, the parts demand of the OEM is based on the principle of "meeting basic functions and low costs". The cost of active balancing technology is much higher than that of passive balancing. When passive balancing meets basic functions, OEMs are more willing to choose passive balancing BMS.

4 Safety Technology The battery management system design should formulate the battery charge and discharge power according to the battery voltage, temperature and the environment in which it is used, and feed the information back to the vehicle so that the battery can be used in a relatively comfortable green area. Batteries are electrochemical carriers and various reactions will occur during charging. In the case of many unsafe factors in the outside world, how to ensure the safety of the battery system is the core issue of battery management technology.