Factors affecting the discharge capacity of lithium-ion battery PACK
Li-ion battery PACK is mainly to test the electrical performance of the cells after screening, grouping, packaging and assembly to determine whether the capacity and pressure difference are qualified products.
The consistency between the series and parallel cells of the battery is a special consideration in the battery pack. Only with good capacity, state of charge, internal resistance, and self-discharge consistency can the battery pack capacity be exerted and released. Poor performance will seriously affect the overall performance of the battery pack, and may even cause overcharge or overdischarge, resulting in safety hazards. A good combination method is an effective way to improve the consistency of monomers.
Lithium-ion batteries are restricted by the influence of ambient temperature, and the battery capacity will be affected if the temperature is too high or too low. If the battery works under high temperature conditions for a long time, its cycle life may be affected. If the temperature is too low, the capacity will be difficult to exert. The discharge rate reflects the high-current charging and discharging capability of the battery. If the rate is too small, the charging and discharging 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 effect and thermal effect of the battery. Charge and discharge rate.
1. Matching Consistency
A good configuration can not only improve the utilization rate of the cells, but also control the consistency of the cells, 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 the battery cell capacity with poor configuration will increase, which in turn will weaken the cycle performance and usable capacity of the battery pack. Someone proposed a method of battery matching according to the characteristic vector of the battery. The characteristic vector reflects the similarity degree between the charge and discharge voltage data of the single battery and the charge and discharge data of the standard battery. The closer the charge-discharge curve of the battery is to the standard curve, the higher the similarity, and the closer the correlation coefficient is to 1. This matching method is mainly based on the correlation coefficient of the monomer voltage, and then combines other parameters to carry out the matching, which can obtain a better matching effect. The difficulty with this approach is to supply standard battery characteristic vectors. Due to production level constraints, there must be differences between each batch of batteries, and it is very difficult to obtain a set of feature vectors that are suitable for each batch of batteries.
Quantitative analysis was used to analyze the difference evaluation method between single cells. First, the key points affecting the battery performance are extracted by mathematical methods, and then mathematical abstraction is carried out to achieve comprehensive evaluation and comparison of battery performance, and the qualitative analysis of battery performance is converted into quantitative analysis, so as to optimize the overall performance of the battery pack. A simple method that can be practically implemented is presented. A comprehensive performance evaluation system based on battery selection and grouping is proposed, combining subjective Delphi scoring and objective gray correlation degree measurement, and establishing a multi-parameter gray correlation model for batteries, which overcomes the one-sidedness of using a single index as the evaluation standard. The performance evaluation of the power lithium-ion battery is realized, and the correlation obtained from the evaluation results provides a reliable theoretical basis for the screening and matching of the battery in the later stage.
The dynamic characteristic matching method is mainly to realize the matching function according to the charging and discharging curve of the battery. The specific implementation steps are to first extract the characteristic points on the curve to form a characteristic vector. According to the distance between the characteristic vectors between each curve, For the matching index, the classification of the curve is realized by selecting an appropriate algorithm, and then the battery matching process is completed. This matching method takes into account the performance changes of the battery during operation. On this basis, other suitable parameters are selected for battery matching, and batteries with more consistent performance can be sorted.
2. Charging method
The appropriate charging regime has a significant impact on the discharge capacity of the battery. If the charging depth is shallow, the discharge capacity will be reduced accordingly. If overcharged, it will affect the chemical active substances of the battery and cause irreversible damage, reducing the capacity and life of the battery. Therefore, it is necessary to choose the appropriate charging rate, upper limit voltage and constant voltage cut-off current to ensure that the charging efficiency and safety and stability are optimized while realizing the charging capacity. At present, power lithium-ion batteries mostly use constant current-constant voltage charging mode. By analyzing the constant current and constant voltage charging results of lithium iron phosphate system and ternary system battery under different charging currents and different cut-off voltages, it can be known that: (1) when the charging cut-off voltage is pressed, the charging current increases and the constant current ratio decreases, The charging time is shortened, but the energy consumption is increased; (2) When the charging current is pressed, as the charging cut-off voltage decreases, the constant current charging ratio decreases, and the charging capacity and energy are both reduced. In order to ensure the battery capacity, iron phosphate The charge cut-off voltage of lithium-ion batteries cannot be lower than 3.4V. To balance charging time and energy loss, choose an appropriate charging current and cut-off time.
The SOC consistency of each cell largely determines the discharge capacity of the battery pack, and balanced charging provides the possibility to achieve a similar initial SOC platform for each cell discharge, which can improve the discharge capacity and discharge efficiency (discharge capacity/matching capacity). The equalization method in charging refers to the equalization of the power lithium-ion battery during the charging process. Generally, the equalization starts when the voltage of the battery pack reaches or exceeds the set voltage, and the overcharge is prevented by reducing the charging current.
According to the different states of the single cells in the battery pack, through the balanced charging control circuit model of the battery pack and the equalization circuit to fine-tune the charging current of the single cells, a method is proposed that can not only realize the rapid charging of the battery pack, but also eliminate the inconsistency of the single cells. Equalizing charge control strategy for battery pack cycle life effects. 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 cell, when the voltage of the single cell reaches a certain value, the balancing module starts to work. The charging current in the single battery is divided to reduce the charging voltage, and the divided current is converted by the module to feed back the energy to the charging bus to achieve the purpose of balance.
Someone proposed a variable rate charging equalization solution. The equalization idea of this method is to only supply additional energy to the single battery with low energy, which prevents the process of extracting the energy of the single battery with more energy, which greatly simplifies the process. The topology of the equalization circuit. That is to say, different charging rates are used to charge the single cells of different energy states, so as to achieve a good balance effect.
3. Discharge rate
The discharge rate is a crucial indicator for power lithium-ion batteries. The high rate discharge of the battery is a test for the positive and negative electrode materials and electrolytes. For the positive electrode material lithium iron phosphate, its structure is stable, the strain during charging and discharging is small, and it has the basic conditions for high 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 an important factor affecting the discharge rate of the battery, and the diffusion of ions in the battery is closely related to the structure of the battery and the concentration of the electrolyte.
Therefore, different discharge rates lead to different discharge time and discharge voltage platforms of the batteries, which in turn lead to different discharge capacities, which are especially obvious for parallel battery packs. Therefore, it is necessary to choose the appropriate discharge rate. The usable capacity of the battery decreases as the discharge current increases.
Jiang Cuina et al. studied the effect of discharge rate on the releasable capacity of lithium iron phosphate battery cells. A group of single cells with good initial consistency of the same type were charged to 3.8V at 1C current, and then charged at 0.1, 0.2, The discharge rates of 0.5, 1, 2, and 3C were discharged to 2.5V, and the relationship curve between the voltage and the discharged power was recorded, as shown in Figure 1. The experimental results show that the released capacity of 1 and 2C is 97.8% and 96.5% of the released capacity of C/3, respectively, and the released energy is 97.2% and 94.3% of the energy released by C/3, respectively. Increase, the capacity and energy released by the lithium-ion battery are significantly reduced.
When the lithium-ion battery is discharged, the national standard 1C is generally used, and the maximum discharge current is usually limited to 2~3C. When a large current is discharged, a large temperature rise will occur 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.
4. Temperature conditions
Temperature significantly affects the activity and electrolyte performance of the electrode material inside the battery. Too high and too low temperature have a greater impact on the capacity of the battery.
At low temperature, the activity of the battery is significantly reduced, the ability of lithium intercalation and extraction is reduced, the internal resistance and polarization voltage of the battery are increased, the actual usable capacity is reduced, the discharge capacity of the battery is reduced, the discharge platform is low, and the battery is more likely to reach the discharge cut-off voltage. As the available capacity of the battery decreases, the energy utilization efficiency of the battery 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 is reduced, and the internal resistance stability time becomes longer, which increases the amount of electron mobility in the external circuit and the capacity is more effective. play. However, if the battery works in a high temperature environment for a long time, the stability of the positive lattice structure will be deteriorated, the safety of the battery will be reduced, and the life of the battery will be significantly shortened.
Li Zhe et al. studied the effect 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. Fit the capacity change of the battery with the temperature, and obtain: In the formula: C is the battery capacity; T is the temperature; R2 is the correlation coefficient of the fitting. Experiments show that the battery capacity decays very quickly at low temperature, while the capacity increases with the increase of temperature at about normal temperature. The capacity of the battery at -40°C is only 1/3 of the nominal value, while at 0°C to 60°C, the battery capacity increases from 80% of the nominal capacity to 100%.
The analysis shows that the change rate of ohmic internal resistance at low temperature is greater than that at high temperature, which indicates that low temperature has a more obvious effect on the activity of the battery, thereby affecting the dischargeable power of the battery. As the temperature rises, the ohmic internal resistance and polarization internal resistance of the charging and discharging process both decrease. However, at higher temperatures, the chemical reaction balance in the battery and the stability of the material will be destroyed, resulting in possible side reactions, which will affect the battery capacity and internal resistance, resulting in shortened cycle life and even reduced safety.
Therefore, both high and low temperatures will affect the performance and service life of lithium iron phosphate batteries. In the actual working process, methods such as new battery thermal management should be used to ensure that the battery works under suitable temperature conditions. In the battery pack PACK test, a constant temperature test room of 25°C can be established.
