Lithium Battery Grouping Inconsistency and Optimization Solution (Ⅱ)

1.Battery cell manufacturing method
Battery Raw Material

The cathode materials of Li-ion batteries include NCM, lithium iron phosphate, lithium cobalt oxide and lithium manganate, and the anode materials include graphite, silicon and lithium titanate. The same batch of raw materials is very important for the consistency of battery performance. In the production process, the particle size distribution, specific surface area and impurity content of raw materials need to be strictly controlled to ensure the batch consistency of raw materials.

2.Production Process of Lithium ion battery
In order to reduce the adverse effect of the initial state difference on the battery pack, it is usually necessary to select the single cell and combine the cells with more consistent state parameters. We call this process as grouping. Battery grouping methods mainly include single cell parameter grouping method, multi parameter grouping method and dynamic characteristic curve grouping method. The dynamic characteristic curve matching method can well reflect the battery characteristics by comparing the differences of different battery charge discharge curves at the same rate, and the separation effect is ideal.

3.External circuit of battery
   Battery Series and Parallel Mode
The connection mode of the battery pack affects the battery consistency. At present, there are two better connection modes: first, parallel two identical batteries into a module, and then connect the modules in series (PSB); First, two different batteries are connected in series to form a module, and then the modules are connected in parallel (SPA).

In order to improve the performance and service life of the battery, it is necessary to manage and maintain the single cell. The battery management system is an important guarantee for the normal operation of the battery system. Its main task is to ensure the performance of the battery pack, prevent battery damage, avoid safety accidents, make the battery work in the appropriate area and extend its life. BMS is composed of sensors, actuators, controllers and signal cables. Its main functions include data acquisition, state estimation, charge and discharge control, equalization charging, heat management, safety management and data communication.

Although battery management technology has been widely used, it still needs to be improved, especially in SOC estimation and data acquisition accuracy, equalization circuit, battery fast charging and so on. Due to the different characteristics of different types of batteries, BMS for all batteries is the main research direction at present.

Equalization Control
In order to alleviate or even eliminate the inconsistencies between individual cells in the battery pack, and improve the performance, life and safety of the battery pack, the inconsistencies of the battery pack can be effectively improved through the equalization circuit and the equalization control strategy.

Topology of equalization circuit: the research of topology of equalization circuit is mainly to design and improve the structure of equalization circuit, improve the efficiency of equalization and reduce the cost. According to whether the equalization circuit consumes energy in the process of equalization, it can be divided into energy consumption equalization and non-energy consumption equalization. The energy consumption equalization circuit uses energy consuming elements to consume the higher voltage of the battery pack, so as to achieve the consistency of the single cell. The circuit is simple, the equalization speed is fast, and the efficiency is high, but it will lead to the low energy utilization rate of the battery pack; Non energy consumption circuit uses energy storage elements and external circuit to realize energy transfer between batteries, which has high energy efficiency. Non energy consumption circuit includes switched capacitor, converter and transformer.

Balancing control strategy: balance control strategy is mainly to determine the working mode of the balance module. At present, there are maximum balance method, average comparison method and fuzzy control method. The improvement of balance ability is an important direction of battery consistency research. Equalization technology needs to be further improved, including:

(1) SOC is the most ideal criterion, and the real-time estimation accuracy needs to be further improved
(2) The topology of equalization circuit is optimized to improve the equalization speed and shorten the equalization time
(3) The equalization control strategy also needs to be optimized to determine the best equalization parameters and find the appropriate equalization path according to the equalization circuit to achieve the purpose of fast equalization.

At present, the research of equalization control strategy mostly focuses on the design and implementation of equalization hardware circuit. But the parameters of equalization circuit will affect the equalization effect. In addition, the battery state of charge, equalization threshold, charging and discharging current, the ratio of equalization current to charging and discharging current, and the switching mode of charging and discharging conditions also affect the equalization effect.

4.Charge and Discharge Strategy
Scientific and reasonable charging and discharging strategy can improve the efficiency of battery energy utilization. At present, the best comprehensive performance charging method is battery management system and charger coordination series charging. Through BMS monitoring the environment temperature of battery pack, voltage and current of single cell, consistency and temperature rise, data sharing is realized with charger, and output current is changed in real time, which can prevent battery overcharge and optimize charging. This kind of charging method is the mainstream at present, which can eliminate the problems of poor consistency, low charging efficiency and unable to fully charge.

5 Battery Thermal Management
The uneven distribution of heat production and heat dissipation of each single cell in the battery pack will cause the temperature inconsistency of the battery itself, some areas of the battery pack and the environment. If not controlled, the internal temperature difference of the battery pack will continue to expand, thus accelerating the degradation of battery performance. Therefore, thermal management of battery pack is needed.

Thermal management system usually requires compact structure, light weight, easy packaging, reliability, low cost and easy maintenance. Its functions are: to make the battery run in the most suitable temperature range; Reduce the temperature difference among batteries, modules and between modules. Thermal management is divided into active and passive ways. There are three kinds of heat conducting media used in the system: air, liquid and phase change materials.

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