Lithium-ion batteries are developing rapidly, and the maximum capacity of lithium-ion battery cells can exceed 300Ah. However, in daily applications, due to the requirements of power and capacity, single lithium-ion cells cannot meet the requirements, so it is necessary to pack the lithium-ion cells, that is, to combine the cells in series and parallel. Finally, a lithium battery pack that fully meets the needs of users is formed.

　　This process seems simple, but in fact it is very sophisticated about the technical level of lithium-ion battery manufacturers, because the inconsistency between single cells often causes the battery pack to experience problems such as rapid capacity decay and short lifespan during the cycle. SES Power, as a manufacturer with profound experience in storing lithium batteries and starting lithium batteries, we choose batteries from well-known brands, because this can make the performance as consistent as possible. Let the senior engineers of SES Power explain to you how we view and deal with inconsistencies in lithium-ion batteries.

　　A, inconsistency analysis

　　a.1 Definition of Inconsistency

　　The inconsistency of lithium-ion battery packs refers to certain differences in parameters such as voltage, capacity, internal resistance, lifespan, temperature influence, and self-discharge rate of each cell after single cells of the same specification and type form a battery pack.

　　There are no identical leaves in the world. After the single cell is manufactured, there are certain differences in the initial performance itself. With the use of the battery, these performance differences continue to accumulate. At the same time, due to the different use environments of the single cells in the battery pack, the inconsistency of the single cells is gradually amplified, thereby accelerating the battery performance degradation and eventually causing the battery pack. Premature failure or even dangerous accidents.

　　a.2 Manifestations of inconsistencies

　　The inconsistency of lithium-ion batteries is mainly manifested in two aspects: differences in battery cell performance parameters (battery capacity, internal resistance and self-discharge rate, etc.) and differences in battery state of charge (SOC).

　　We found that the difference distribution of capacity between battery cells is close to the Weir distribution, while the dispersion of internal resistance is more significant than that of capacity, and the internal resistance of the same batch of batteries generally satisfies the law of normal distribution, and the self-discharge rate also presents approximately normal distribution.

　　SOC represents the state of charge of the battery and is the ratio of the remaining capacity of the battery to the rated capacity. Due to the inconsistency of the battery, the capacity decay rate of the battery is different, resulting in the difference in the maximum available capacity between the batteries. The SOC change rate of the battery with small capacity is faster than that of the battery with large capacity, and the cut-off voltage is reached faster during charging and discharging.

　　a.3 Causes of Inconsistency

　　There are many reasons for the inconsistency of lithium-ion batteries, mainly in the manufacturing process and the use process: each link of the manufacturing process, such as the uniformity of the slurry during batching, the control of surface density and surface tension during coating, etc. Causes differences in the performance of single cells.

　　The water-based binder system, connection method, structural parts/devices, operating conditions and environment will all have an impact on 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 positions of each single cell in the battery, different temperatures, and different performance attenuation, these will amplify the inconsistency of the single cells.

　　B. Methods to improve battery consistency

　　b.1 Control of production process

　　The control of the production process is mainly carried out from two aspects: raw materials and production process. 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 should be strictly controlled, such as ensuring that the slurry is evenly stirred, not placed for a long time, and controlling the coating machine. The running speed ensures the thickness and uniformity of the coating, the appearance inspection of the pole piece, the weighing and grading, and the control of the injection volume and chemical composition, volume distribution, storage conditions, etc.

　　The key processes that have a significant impact on the consistency of lithium-ion batteries are mainly ingredient stirring, coating, rolling, winding/lamination, liquid injection and formation.

　　b.2 Control of the matching process

　　This is a very critical step and an important part of testing the capacity of the lithium battery pack factory! The control of the grouping process mainly refers to the sorting of the batteries, the batteries of the same specification and type are used for the battery group, and the voltage, capacity, internal resistance, etc. of the batteries should be measured to ensure the consistency of the initial performance of the batteries.

　　When the battery pack is assembled, the voltage difference of the single cell is an important factor affecting the consistency of each single cell at the end of the charge and discharge of the battery pack, while the internal resistance difference of the single cell causes the battery pack to be charged and discharged. There is a big difference in the voltage platform of the battery.

　　Through the research on the inconsistency of single cells in lithium-ion series-parallel combined batteries, SES Power analyzes that the main influencing factor in parallel battery packs is DC internal resistance, and the main influencing factor in series-connected battery packs is capacity difference. At the same time, with the increase of the discharge rate, the inconsistency of the battery is amplified.

　　b.3 Control of the use and maintenance process

　　It is very important to monitor the battery in real time, and it is also the main reason why SES Power gradually reduces the non-intelligent PMC and adopts the higher cost BMS with RS485 or CANBus communication function.

　　Because the consistency screening of the batteries is carried out when matching, the consistency can only be guaranteed in the initial use of the battery pack. The consistency of the battery will be gradually enlarged with the increase of the use time, so it is necessary to monitor the battery in real time during the use process, and observe the consistency problem during the use process in real time. Replacement ensures that battery pack inconsistencies do not expand over time.

　　Of course, the introduction of a balanced management system is the optimal solution, provided that the cost and absolute stability are not considered. At present, common balancing strategies include external voltage-based balancing strategies, SOC-based balancing strategies, and capacity-based balancing strategies. The equalization circuit can be divided into passive equalization and active equalization according to the energy consumption mode. Active balancing can achieve lossless energy flow between cells, which is a method highly recommended by SES Power, but the high cost limits its application. Commonly used methods in active equalization include battery bypass method, switched capacitor method, switched inductor method, and DC/DC conversion method.

　　SES Power has applied the active balancing scheme in the 1Kwh-10Kwh lithium iron phosphate battery energy storage system. We simulated the situation when the battery is seriously inconsistent and found that it can truly solve the problem of inconsistency, so that the lithium battery can truly the ten-year application period is reached.

　　The thermal management of the battery pack is also an important part. In addition to trying to keep the operating temperature of the battery pack within the optimal range, it is also necessary to try to ensure that the temperature conditions between the batteries are consistent, so as to effectively ensure the performance of each battery. consistency.

　　Finally, a reasonable control strategy needs to be adopted. When the output power allows, try to reduce the depth of discharge of the battery, and at the same time, avoid overcharging of the battery, which can prolong the cycle life of the battery pack; strengthen the maintenance of the battery pack; conduct low-current maintenance charging of the battery pack at certain intervals, but also pay attention to cleaning.

　　C. Lithium-ion battery matching method

　　c.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, only considers the battery itself, and measures the self-discharge rate and open circuit voltage of the selected single battery after standing for a certain period of time. precise. The dynamic voltage matching method examines the voltage situation with load, but does not take into account factors such as load changes, so it is not accurate.

　　c.2 Static capacity allocation method

　　The battery is charged and discharged under the set conditions, the capacity is calculated from the discharge current and discharge time, and the batteries are grouped according to the capacity. This method is simple and easy to implement, but it can only reflect the same capacity of the battery under specific conditions, and cannot explain the complete working characteristics of the battery, and has certain limitations.

　　c.3 Internal resistance matching method

　　Mainly considering the internal resistance of the single cell, this method enables fast measurement. But SES Power must remind that the AC internal resistance and the DC internal resistance are different. For small-capacity cells, we can think that the AC internal resistance is equal to the DC internal resistance, but for large-capacity cells, the AC internal resistance is only for reference. , the DC internal resistance is decisive. In addition, the internal resistance of the battery will change with the discharge process, and it is difficult to accurately measure the internal resistance.

　　c.4 Multi-parameter matching method

　　At the same time, considering multiple external conditions such as capacity, internal resistance, voltage, and self-discharge rate, the battery can be comprehensively evaluated, and the battery pack with better consistency can be sorted. However, the premise of this method is that the single-parameter sorting should be accurate, and it will take too long.

　　c.5 Dynamic characteristic matching method

　　The dynamic characteristic matching method is to use the charge-discharge characteristic curve of the battery to sort the batteries for matching. The charge-discharge curve can reflect most of the characteristics of the battery, and the dynamic characteristics matching method can ensure the consistency of various performance indicators of the battery.

　　The dynamic characteristic matching method has a lot of data, and is usually realized by computer programs. In addition, this method reduces the utilization rate of the battery pack, which is not conducive to the reduction of the cost of the battery pack. The determination of standard curve or reference curve is also a difficult point in its implementation.

　　SES Power has developed and produced many lithium-ion batteries for different applications during its nearly 20-year-long application career of lithium-ion batteries. 3kwh, 5kwh, 10kwh wall-mounted smart home energy storage system, rack-mounted energy storage system, split combined high-voltage power lithium battery system, polar lithium battery that can withstand -60 degrees Celsius, starting battery with starting current up to 2000CCA, customized Metalized metal shell lithium iron phosphate battery, etc., we finally summarize for you:

　　1. The causes of battery inconsistency are mainly in the manufacture and use of batteries.

　　2. The measures to improve the consistency of batteries mainly include the following three aspects: strictly control the production process from both raw materials and production processes; adopt a more scientific sorting method, and select batteries with the same initial performance as possible for grouping; In the process of battery use and maintenance, the battery is monitored in real time, a balanced management system is introduced, and a reasonable control strategy is adopted to conduct thermal management of the battery, and at the same time, strengthen the maintenance of the battery pack.

　　3. When the battery is matched, the single-parameter matching method does not have practical application value because too few factors are considered. The multi-parameter matching method and the dynamic characteristic matching method are relatively comprehensive.