Lithium-ion batteries have the advantages of large capacity, high specific energy, good cycle life, no memory effect, etc., and they are developing rapidly. The capacity as its most critical performance index has also attracted researchers' attention. Correspondingly, the lithium battery PACK is constantly developing in the direction of large capacity, fast charging, long life and high safety, and new requirements are also put forward for the process technology in its manufacturing process.
Lithium-ion battery PACK is mainly a product whose electric core is screened, assembled, packaged and assembled to determine whether the capacity and pressure difference are qualified.
The consistency between battery series and parallel cells needs special consideration in the battery PACK. Only with good capacity, state of charge, internal resistance, self-discharge consistency, etc. can the battery pack capacity be exerted and released. Poor performance will seriously affect the overall performance of the battery pack, and even cause overcharging or overdischarging to cause safety hazards. A good matching scheme is an effective way to improve monomer consistency.
Lithium-ion batteries are affected by the environmental temperature, too high or too low temperature will affect the battery capacity. The cycle life of the battery may be affected if it is operated under high temperature conditions for a long time. If the temperature is too low, the capacity will be difficult to play.
The discharge rate reflects the high current charge and discharge capacity of the battery. If the rate is too small, the charge and discharge speed will be slow, which will affect the test efficiency; if the rate is too large, the capacity will be reduced due to the polarization and thermal effects of the battery, so you need to choose a suitable one. Charge and discharge rate.
1. Matching group consistency
A good combination can not only improve the utilization rate of the cells, but also control the consistency of the monomers, which is the basis for achieving good discharge capacity and cycle stability in the discharge of the battery pack. However, the dispersion of the AC impedance of a poorly matched battery cell capacity will increase, which in turn will weaken the cycle performance and usable capacity of the battery pack.
Professionals have proposed a method of battery matching according to the feature vector of the battery. This feature vector reflects the similarity between the charge and discharge voltage data of the single battery and the charge and discharge data of the standard battery. The closer the battery charge and discharge curve is to the standard curve, the higher the degree of similarity, and the closer the correlation coefficient is to 1. This kind of grouping method is based on the correlation coefficient of the cell voltage, and then combined with other parameters for grouping, which can get a better grouping effect. The difficulty of this method is that it needs to provide a standard battery feature vector. Due to the limitation of the production level, there must be differences between the batteries produced in each batch, and it is very difficult to obtain a set of feature vectors suitable for each batch of batteries.
Professionals used quantitative analysis to analyze the difference evaluation methods between single cells. First, use mathematical methods to extract the key points that affect battery performance, and then perform mathematical abstraction to achieve comprehensive evaluation and comparison of battery performance, transform qualitative analysis of battery performance into quantitative analysis, and best match the overall performance of the battery pack A simple method that can be operated in practice is proposed. A comprehensive performance evaluation system based on battery screening and matching is proposed, which combines subjective Delphi scoring and objective gray correlation measurement to establish a multi-parameter gray correlation model for batteries, which overcomes the one-sidedness of using a single indicator as the evaluation standard. The performance evaluation of the power-type power battery is realized, and the correlation degree obtained from the evaluation result provides a reliable theoretical basis for the later selection and assembly of the battery.
The dynamic characteristic matching method is mainly based on the battery charging and discharging curve to realize the matching function. The specific implementation step is to first extract the characteristic points on the curve to form a characteristic vector. According to the distance between the characteristic vectors of each curve, The allocation index is to realize the classification of the curve by selecting the appropriate algorithm, and then complete the battery allocation process. This kind of grouping method considers the performance changes of the battery during operation. On this basis, other suitable parameters are selected for battery matching, and batteries with relatively consistent performance can be sorted out.
2. Charging method
A proper charging system has an important influence on the discharge capacity of the battery. If the charge depth is shallow, the discharge capacity will be reduced accordingly. If overcharged, it will affect the chemical active materials of the battery and cause irreversible damage, reducing the capacity and life of the battery. Therefore, it is necessary to select an appropriate charging rate, upper limit voltage, and constant voltage cut-off current to ensure that the charging capacity is achieved while optimizing charging efficiency and safety and stability.
At present, the power lithium-ion battery mostly adopts the constant current-constant voltage charging mode. Liu Yanjin et al. analyzed the constant current and constant voltage charging results of lithium iron phosphate system and ternary system batteries at different charging currents and different cut-off voltages, and found that: (1) When the charging cut-off voltage is constant, the charging current increases and the constant current ratio decreases. When the charging current is small, the charging time is shortened, but the energy consumption is increased; (2) When the charging current is constant, as the charging cut-off voltage decreases, the constant current charging ratio decreases, and the charging capacity and energy are decreasing. In order to ensure the battery capacity, phosphoric acid The charge cut-off voltage of the lithium iron battery cannot be lower than 3.4V. It is necessary to balance the charging time and energy loss, and select the appropriate charging current and cut-off time.
The consistency of the SOC of each cell determines the discharge capacity of the battery pack to a large extent, and the equalization charge provides the possibility to achieve the similarity of the initial SOC platform of each cell discharge, which can improve the discharge capacity and discharge efficiency (discharge capacity/matched group capacity). The balancing method in charging refers to the balancing of the power battery during the charging process. Generally, the balancing starts when the battery cell voltage reaches or exceeds the set voltage, and prevents overcharging by reducing the charging current.
According to the different states of the single cells in the battery pack, professionals use the battery pack equalization charge control circuit model and the equalization circuit to fine-tune the charging current of the single cells, and propose a method that can realize the rapid charging of the battery pack and eliminate the single cells. Inconsistent balancing charge control strategy affecting the cycle life of the battery pack. Specifically, through the switch signal, the overall energy of the lithium-ion battery pack is supplemented to the single battery, or the energy of the single battery is converted into the overall battery pack. During the charging process of the battery pack, by detecting the voltage value of each single battery, when the voltage of the single battery reaches a certain value, the equalization module starts to work. The charging current in the single battery is shunted to reduce the charging voltage, and the divided current is converted by the module and the energy is fed back to the charging bus to achieve the purpose of balance.
Professionals have proposed a variable rate charging equalization solution. The equalization idea of the solution is to only supplement the low-energy single battery with additional energy, avoiding the process of taking out the energy of the high-energy single battery, which greatly simplifies The topology of the equalizing circuit is described. That is, different charging rates are used to charge single cells in different energy states, so as to achieve a good balance effect.
3. Discharge rate
The discharge rate is an important indicator for power-type power batteries. The high-rate discharge of the battery is a test for the positive and negative materials and electrolyte. For the positive electrode material lithium iron phosphate, its structure is stable, the strain during charge and discharge is small, and it has the basic conditions for large current discharge, but the disadvantage is that the conductivity of lithium iron phosphate is poor. The diffusion rate of lithium ions in the electrolyte is the main factor that affects the discharge rate of the battery, and the diffusion of ions inside the battery is closely related to the structure of the battery and the concentration of the electrolyte.
Therefore, different discharge rates result in different discharge times and discharge voltage platforms of the batteries, which in turn lead to different discharge capacities, which is especially obvious for parallel battery packs. Therefore, it is necessary to select an appropriate discharge rate. The relationship between discharge capacity and discharge rate (current) can be described by Peukert's equation:
C=KII-n
In the formula: I is the discharge current; n is the Peukert constant, which is related to the structure of the battery, and its value is between 1.15 and 1.42; K is a constant, which is a constant related to the amount of active material in the battery.
The available capacity of the battery decreases as the discharge current increases.
Professionals have studied the effect of discharge rate on the discharge capacity of lithium iron phosphate battery cells. A group of single cells with the same model with good initial consistency are charged to 3.8V with a current of 1C, and then charged with 0.1, 0.2, and 0.5 respectively. , 1, 2, and 3C discharge rate to 2.5V, record the relationship curve between the voltage and the discharged electricity. The experimental results show that the discharge capacity of 1 and 2C are respectively 97.8% and 96.5% of the discharge capacity of C/3, and the energy released are 97.2% and 94.3% of the energy discharged by C/3, respectively. Increasing, the capacity and energy released by the lithium-ion battery have been significantly reduced.
When discharging lithium-ion batteries, the national standard 1C is generally used, and the maximum discharge current is usually limited to 2 to 3C. When discharging with high current, it will produce a large temperature rise and lead to energy loss. Therefore, it is necessary to monitor the temperature of the battery pack in real time to prevent damage to the battery due to excessive temperature and reduce the service life of the battery.
Fourth, temperature conditions
Temperature mainly affects the activity and electrolyte performance of the pole piece material inside the battery. Too high and too low temperature have a greater impact on the capacity of the battery.
At low temperatures, the battery’s activity is significantly reduced, the ability to insert and extract lithium decreases, the internal resistance and polarization voltage of the battery increase, the actual available capacity decreases, the battery discharge capacity decreases, the discharge platform is low, and the battery is more likely to reach the discharge cut-off voltage. The available capacity of the battery decreases, and the efficiency of battery energy utilization decreases.
When the temperature rises, the extraction and insertion of lithium ions between the positive and negative electrodes become active, so that the internal resistance of the battery decreases, and the internal resistance stabilization time becomes longer, which makes the amount of electron migration in the external circuit increase, and the capacity is more effective. Play. However, if the battery is operated in a high temperature environment for a long time, the stability of the positive electrode lattice structure will be deteriorated, the safety of the battery will be reduced, and the battery life will be significantly shortened.
Professionals have studied the influence of temperature on the actual discharge capacity of the battery, and recorded the ratio of the actual discharge capacity of the battery to the standard discharge capacity (1C discharge at 25°C) at different temperatures. Fitting the change of battery capacity with temperature, we get:
C=(-5.06974)*exp(-T/55.90333)+14.03729,R2-0.99784
In the formula: C is the battery capacity; T is the temperature; R2 is the correlation coefficient of the fitting. Experiments have shown that the battery capacity decays extremely fast at low temperatures, while the capacity increases with the increase in temperature around room temperature. The battery capacity is only 1/3 of the nominal value at -40°C, while at 0°C to 60°C, the battery capacity rises from 80% of the nominal capacity to 100%.
The analysis believes that the rate of change of ohmic resistance at low temperature is greater than that at high temperature, which indicates that low temperature has a significant impact on battery activity, thereby affecting the battery's discharge capacity. As the temperature rises, the ohmic internal resistance and polarization internal resistance both decrease during charging and discharging. However, at higher temperatures, the balance of chemical reactions in the battery and the stability of the materials will be destroyed, and side reactions may occur, which will affect the battery capacity and internal resistance, resulting in shortened cycle life and even reduced safety.
Therefore, both high temperature and low temperature will affect the performance and service life of the lithium iron phosphate battery. In the actual working process, methods such as increasing battery thermal management should be used to ensure that the battery works under suitable temperature conditions. In the battery pack PACK test link, a constant temperature testing room at 25°C can be established.
Five, summary
In this paper, combined with the actual situation of lithium-ion battery PACK, the factors that affect the discharge capacity are analyzed and discussed. Good battery pack matching group consistency is the prerequisite for realizing battery pack discharge performance and level, you can refer to the use of dynamic characteristics matching group method. The charging method is recommended to use the balanced charging method to ensure that the SOC platforms of each monomer are similar before discharging. It is necessary to choose a suitable discharge rate, taking into account both capacity and test efficiency. The environment has a great influence on battery testing, so temperature conditions need to be controlled.
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