Due to the lithium ion batteries have the advantages of large capacity, high specific energy, good cycle life, no memory effect and so on, so lithium batteries are developing very quickly. Capacity, as the most critical performance index, has attracted much attention. Accordingly, lithium battery pack is developing towards large capacity, fast charging, long life and high safety, and new requirements are put forward for its manufacturing process.
Lithium battery pack is mainly a product that conducts electrical performance test after sorting, assembling and grouping the cells to determine whether the capacity and voltage difference are qualified.
The consistency of the battery is the most important in the battery pack .Only when the capacity, state of charge, internal resistance and self-discharge have good consistency, can the battery pack capacity release well. If the consistency is poor, the overall performance of the battery pack will be seriously affected, and even lead to overcharge or over-discharge, resulting in safety risks. A good configuration scheme is an effective way to improve the consistency of single cell. For the 72V battery in United States ,Especially 72V 200Ah lithium battery, 72V 100Ah lithium battery pack and other lithium ion golf cart batteries; 48V lithium ion battery, especially 48V 100Ah lithium battery, 48V 300Ah lithium battery for AGV have higher requirements for the battery consistency.
Lithium ion battery is restricted by the ambient temperature, too high or too low temperature will affect the battery capacity. The cycle life of the battery may be affected if the battery works at high temperature for a long time. If the temperature is too low, the capacity will be difficult to release.
The discharge rate reflects the battery’s capacity of charging and discharging at high current. If the discharge rate is too small, the charging and discharging speed is slow, which affects the test efficiency. If the rate is too large, the capacity of the battery will be reduced due to the polarization effect and thermal effect, so it is necessary to choose an appropriate charge and discharge rate.
Good configuration can not only improve the utilization rate of the cell, but also control the consistency of the cell, which is the basis of achieving good discharge capacity and cycle stability of the battery pack. However, the dispersion of ac impedance of battery cell capacity will be aggravated due to poor configuration, which will weaken the cycle performance and available capacity of battery pack.
At present, the most commonly used configuration method is to determine the capacity difference, voltage difference and internal resistance difference at a specific value as the basis of battery configuration, such as capacity difference < 30mAh, voltage difference < 5mV, internal resistance difference < 3 mΩ.
Appropriate charging system has an important effect on the discharge capacity of batteries. If under charge, the discharge capacity will decrease correspondingly. If over charge, the chemical active substances of the battery will be affected and irreversible damage will be caused, reducing the capacity and life of the battery. Therefore, it is necessary to select appropriate charging rate, upper voltage and constant voltage cutoff current to ensure the optimization of charging efficiency and safety and stability while realizing the charging capacity.
At present, the power lithium ion battery mostly adopts CC(constant-current) – CV(constant-voltage) charging mode. By analyzing the CC and CV charging results of lithium iron phosphate system and NCM ternary system batteries under different charging currents and different cutoff voltages, it can be seen that :
01. when the cut-off voltage is constant, the charging current increases, the constant current ratio decreases, the charging time decreases, but the energy consumption increases;
02. When the charging current is constant, with the decrease of the charging cut-off voltage, the constant current charging ratio decreases, and the charging capacity and energy both decrease. In order to ensure the battery capacity, the charging cut-off voltage of the lithium iron phosphate battery should not be lower than 3.4V. It is necessary to balance charging time and energy loss, and choose appropriate charging current and cut-off time.
The consistency of SOC of each cell largely determines the discharge capacity of battery pack, and balanced charging provides the possibility to realize the similarity of initial SOC platform of each cell discharge, which can improve the discharge capacity and discharge efficiency (discharge capacity/configuration capacity). The balancing mode in charging refers to the balancing of the power battery in the charging process. It generally starts to balance when the voltage of the battery reaches or is higher than the set voltage, and prevents overcharging by reducing the charging current.
According to the different states of the single cell in the battery pack, a balanced charging control strategy was proposed to realize the quick charging of the battery pack and eliminate the influence of the inconsistent single cell on the cycle life of the battery pack by adjusting the charging current of the single cell through the balanced charging control circuit model. Specifically, the overall energy of the lithium ion battery pack is supplemented to the single cell by switching signals, or the energy of the single cell is converted to the overall battery pack. When the voltage of each single cell reaches a certain value, the balancing module starts to work. The charging current in the single cell is shunt to reduce the charging voltage, and the energy is fed back to the charging bus through the module for conversion, so as to achieve the purpose of balance.
Discharge rate is a very important index for power battery. The large discharge rate of the battery is a test for positive and negative electrode materials and electrolyte. As for lithium iron phosphate, it has stable structure, small strain during charge and discharge, and has the basic conditions of large current discharge, but the unfavorable factor is the poor conductivity of lithium iron phosphate. The diffusion rate of lithium ion in electrolyte is the main factor affecting the discharge rate of battery, and the diffusion of ion in battery is closely related to the structure and electrolyte concentration of battery.
Therefore, different discharge rates lead to different discharge time and discharge voltage platforms of batteries, which leads to different discharge capacities, especially for parallel batteries. Therefore, appropriate discharge rate should be selected. The relation between discharge capacity and discharge rate (current) can be described by Peukert equation C=KI(1-n).
We study the discharge rate of lithium iron phosphate battery cells is the influence of the discharge capacity, a set of the same type of initial consistency better monomer battery are in 1 c current charge to 3.8 V, then respectively by 0.1, 0.2, 0.5, 1, 2, 3 c discharge rate of discharge to 2.5 V, record the relationship between the voltage and discharge power curve, 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, and the released energy is 97.2% and 94.3% of the released energy of C/3, respectively. It can be seen that with the increase of discharge current, the released capacity and the released energy of lithium ion battery decrease significantly.
In the discharge of lithium ion batteries, the national standard 1C is generally selected, and the maximum discharge current is usually limited to 2 ~ 3C. When discharging with high current, the temperature rise will be higher and the energy will be lost. Therefore, monitor the temperature of battery strings in real time to prevent battery damage and shorten the battery life.
Temperature mainly affects the activity of electrode material and electrolyte performance. The battery capacity is greatly affected by high or low temperature.
At low temperature, the activity of the battery is significantly reduced, the ability of embedding and releasing lithium decreases, the internal resistance of the battery and the polarization voltage increase, the actual available capacity is reduced, the discharge capacity of the battery is reduced, the discharge platform is low, the battery is easier to reach the discharge cut-off voltage, which is manifested as the battery available capacity decreases, the battery energy utilization efficiency decreases.
As the temperature rises, the lithium ions emerge and embed between the positive and negative poles become active, so the battery internal resistance decreases and the stabilization time of internal resistance becomes longer, which increases the electronic bande movement in the external circuit and makes the capacity more effective. However, if the battery works at high temperature for a long time, the stability of the positive lattice structure will become worse, the safety of the battery will be reduced, and the life of the battery will be significantly shortened.
A large number of experiments show that the capacity of the battery decays very fast at low temperature, while the capacity increases with the increase of temperature at room temperature. The capacity of the battery at -40℃ is only one-third of the nominal value, while at 0℃ to 60℃, the capacity of the battery rises from 80% of the nominal capacity to 100%
The analysis shows that the rate of change of resistance at low temperature is greater than that at high temperature, which indicates that the low temperature has a significant impact on the activity of the battery, thus affecting the battery can be released. With the increase of temperature, the resistance and polarization resistance of charging and discharging process decrease. However, at higher temperatures, the chemical reaction balance and material stability in the battery will be destroyed, resulting in possible side reactions, which will affect the capacity and internal resistance of the battery, resulting in shortened cycle life and even reduced safety.
Therefore, both high temperature and low temperature will affect the performance and service life of lithium iron phosphate battery, and the battery thermal management method should be adopted to ensure that the battery works at an appropriate temperature. A constant temperature testing room of 25℃ can be established in the battery PACK test link.
In this paper, the factors affecting the discharge capacity of lithium ion battery PACK are analyzed and discussed. When lithium battery customize, good consistency of battery configuration is the premise to achieve the discharge performance and level of battery pack, and dynamic characteristic configuration method can be used as a reference. Charging mode You are advised to use a balanced charging mode to ensure that each SOC platform is similar before discharging. It is necessary to choose a suitable discharge rate and give consideration to both capacity and test efficiency. The environment has great influence on the battery test, so the temperature condition should be well controlled.