How to improve the consistency of lithium batteries
New energy vehicles are becoming more and more popular. When lithium batteries are used as the power source of electric vehicles, due to the requirements of high power and large capacity, single lithium ion batteries cannot meet the requirements, so lithium ion batteries need to be connected in series and parallel. Used in combination.
However, the inconsistency between the single cells often causes problems such as excessive capacity decay and short life of the battery pack during the cycle. Choosing batteries with as consistent performance as possible for grouping is of great significance to the promotion and application of lithium-ion batteries in power batteries. Now make a simple analysis from several aspects:
1 Inconsistency analysis
1.1 Definition of inconsistency The inconsistency of a lithium-ion battery pack refers to a certain difference in parameters such as voltage, capacity, internal resistance, life, temperature influence, and self-discharge rate after the single cells of the same specification and model form a battery pack. After the single battery is manufactured, there is a certain difference in the initial performance itself. With the use of batteries, these performance differences continue to accumulate. At the same time, because the use environment of each single battery in the battery pack is not exactly the same, it also causes the inconsistency of the single battery to gradually enlarge, thereby accelerating the degradation of battery performance, and finally causing The battery pack fails prematurely. 1.2 Performance of inconsistency The inconsistency of lithium-ion batteries is mainly manifested in two aspects: the difference in battery cell performance parameters (battery capacity, internal resistance and self-discharge rate, etc.) and the difference in battery state of charge (SOC). Dai Haifeng et al. found that the distribution of the difference in capacity between battery cells is close to the Weir distribution, and the dispersion of internal resistance is more significant than that of capacity, and the internal resistance of the same batch of batteries generally meets the law of normal distribution, self-discharge The rate also presents an approximately normal distribution. SOC characterizes the state of charge of the battery, which is the ratio of the remaining capacity of the battery to the rated capacity. Jie Jing et al. believe that due to the inconsistency of the battery, the capacity decay rate of the battery is different, resulting in a difference in the maximum usable capacity between the batteries. The SOC change rate of a battery with a small capacity is faster than that of a battery with a large capacity, and the cut-off voltage is reached faster during charging and discharging.
1.3 Causes of inconsistency There are many reasons for the inconsistency of lithium-ion batteries, mainly in the manufacturing process and use process. Every aspect of the manufacturing process, such as the uniformity of the slurry during the batching, the control of the areal density and surface tension during coating, etc., will cause the difference in the performance of the single cell. Luo Yu et al. studied the impact of lithium-ion battery production and manufacturing processes on battery consistency, and focused on the impact of water-based binder system lithium-ion battery production process on battery consistency. During the use of the battery, Xie Jiao and others believe that the connection method, structural parts/devices, operating conditions and environment will all affect the consistency of the battery pack. Because the energy consumed by each connection point is inconsistent, the performance and aging rate of each component or structure are also inconsistent, so the impact on the battery is also inconsistent. In addition, due to the different location of each single cell in the battery, different temperature, and different performance degradation, these will amplify the inconsistency of the single cell.
2 Ways to improve battery consistency
2.1 Control of the production process The control of the production process is mainly carried out from two aspects: raw materials and production processes. In terms of raw materials, try to select the same batch of raw materials to ensure the consistency of the particle size and performance of the raw materials. In the production process, the entire production process must be strictly controlled, such as ensuring that the slurry is evenly stirred and not placed for a long time, controlling the speed of the coating machine to ensure the thickness and uniformity of the coating, the appearance of the pole piece, and the weighing and classification. , Control the injection volume, formation, volume separation, storage conditions, etc. Luo Yu has determined the key processes that have a significant impact on the consistency of lithium-ion batteries through research on the preparation technology of lithium-ion batteries, including batching mixing, coating, rolling, winding/lamination, liquid injection and formation. In-depth research and analysis of the relationship between the key process parameters and battery performance are also done.
2.2 Control of the configuration process
The control of the assembly process mainly refers to the sorting of batteries. The battery pack adopts batteries of uniform specifications and models, and the voltage, capacity, internal resistance, etc. of the batteries must be measured to ensure the consistency of the initial performance of the batteries. Through research, Xu Haitao et al. found that when the battery pack is assembled, the voltage difference of the single cells is an important factor affecting the consistency of the single cells at the end of the battery pack’s charge and discharge. The difference in the internal resistance of the single cells causes the battery pack to charge During the discharging process, the voltage platform of each single battery is quite different. Wang Linxia and others analyzed the inconsistency of single cells in lithium-ion series-parallel battery packs, and analyzed the main influencing factors in parallel battery packs. The degree of influence of the battery pack provides the necessary basis for the assembled battery pack. Chen Ping et al. studied the influence of the discharge rate on the consistency of the battery configuration and found that with the increase of the discharge rate, the inconsistency of the battery has been amplified, achieving the effect of eliminating bad batteries.
2.3 Use and maintenance process control to monitor the battery in real time. The consistency of the battery is screened when the battery is assembled, which can ensure the consistency of the battery in the initial stage of use. The battery is monitored in real time during use, and consistency problems during use can be observed in real time. However, when the consistency is poor, the monitoring circuit will cut off the charging and discharging circuit, and the performance will be reduced. A balance between the two must be found. The extreme parameter battery can also be adjusted or replaced in time through real-time monitoring to ensure that the inconsistency of the battery pack will not expand over time. Introduce a balanced management system. Adopt appropriate equalization strategy and equalization circuit to intelligently manage the battery. The current common balancing strategies include the balancing strategy based on external voltage, the balancing strategy based on SOC and the balancing strategy based on capacity. The equalization circuit can be divided into passive equalization and active equalization according to the way of energy consumption. Among them, active equalization can realize lossless energy flow between batteries, which is a hot research topic at home and abroad. Commonly used methods in active balancing include battery bypass method, switched capacitor method, switched inductance method, and DC/DC conversion method.
Thermal management of the battery. In addition to keeping the operating temperature of the battery pack within the optimal range, thermal management of the battery should also try to ensure the consistency of the temperature conditions between the batteries, so as to effectively ensure the consistency of performance between the batteries. Use reasonable control strategies. When the output power allows, try to reduce the depth of battery discharge, and at the same time, avoid overcharging of the battery, which can extend the cycle life of the battery pack. Strengthen the maintenance of battery packs. Perform low-current maintenance charging on the battery pack at regular intervals, and pay attention to cleaning.
3 Assembling method of power lithium-ion battery
3.1 Voltage matching method The voltage matching method can be divided into static voltage matching method and dynamic voltage matching method. The static voltage matching method is also called the no-load matching method. It does not carry a load and only considers the battery itself. It measures the self-discharge rate of the fully charged state of the selected single battery after tens of days of standing and the different storage periods in the fully charged state. The open circuit voltage of the internal battery, this method is the simplest operation, but it is not accurate. The dynamic voltage matching method investigates the voltage situation with load, but it does not take into account factors such as load changes, so it is not accurate.
3.2 The static capacity matching method charges and discharges the battery under the set conditions, calculates the capacity from the discharge current and discharge time, and matches the battery according to the capacity. This method is simple and easy to implement, but it can only reflect that the battery has the same capacity under certain conditions, and cannot explain the complete working characteristics of the battery, and has certain limitations.
3.3 The internal resistance matching method mainly considers the internal resistance of the single battery. This method can achieve rapid measurement, but because the internal resistance of the battery will change with the discharge process, it is difficult to accurately determine the internal resistance.
3.4 The multi-parameter matching method simultaneously considers the capacity, internal resistance, voltage, self-discharge rate and other external conditions to comprehensively evaluate the battery, and can sort out the battery pack with better consistency. However, the premise of this method is that the single-parameter sorting must be accurate and time-consuming.
3.5 Dynamic characteristic grouping method The dynamic characteristic grouping method uses the charge and discharge characteristic curve of the battery to sort the batteries for grouping. The charge-discharge curve can reflect most of the characteristics of the battery, and the use of dynamic characteristics matching method can ensure the consistency of various performance indicators of the battery. There are many data in the dynamic characteristics matching method, which is usually realized by the cooperation of computer programs. In addition, this method reduces the battery pack utilization rate, which is not conducive to the reduction of battery composition cost. The determination of the standard curve or reference curve is also a difficult point in its implementation. 4 Conclusion
The reason for the inconsistency of the battery is mainly in the manufacture and use of the battery.
The measures to improve battery consistency mainly include the following three aspects:
1. Strictly control the production process from two aspects of raw materials and production technology;
2. Use a more scientific sorting method, and try to select batteries with the same initial performance for grouping;
3. In the process of battery use and maintenance, monitor the battery in real time, introduce a balanced management system, adopt a reasonable control strategy, conduct thermal management of the battery, and strengthen the maintenance of the battery pack.
