Demystifying The Impact Of Lithium Battery PACK Discharge Capacity

  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.

1. Matching group consistency

  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.

  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.

 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 simultaneously optimizing charging efficiency and safety and stability.

  At present, the power lithium-ion battery mostly adopts the constant current-constant voltage charging mode. By analyzing 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, it can be seen that: (1) When the charging cut-off voltage is constant, 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 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, lithium iron phosphate The charge cut-off voltage of the 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.

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.

  Different discharge rates lead to 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 discharge curve of LiFePO4 power battery at different rates of 0.1 C, 0.2 C, 0.5 C, 1 C, 1.5 C and 2 C at an ambient temperature of 25 ℃. It can be seen that the discharge process of LiFePO4 battery at different rates has a stable Discharge platform, the platform voltage is between 3.0~3.4 V. In addition, the platform voltage decreases with the increase of the discharge rate. This is because the increase of the discharge rate increases the discharge current of the battery and the internal resistance of the battery. , The voltage will increase when the battery is discharged. On the other hand, the high-rate current discharge will increase the polarization of the battery and reduce the voltage platform of the battery.

  The discharge capacity of LiFePO4 battery under different discharge rate will decrease with the increase of battery discharge rate. At a small rate of 0.1 C, the discharged capacity of the battery is larger than the nominal capacity of the battery, but at a rate of 2 C The discharge capacity is only 90% of the nominal capacity. This shows that there is a certain negative correlation between the discharge rate and the capacity of the battery.

  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.

4. Temperature conditions

  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.

  Temperature mainly affects the activity and electrolyte performance of the pole piece material inside the battery. At low temperatures, the activity of the battery decreases significantly, 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 deteriorate, the safety of the battery will be reduced, and the battery life will be significantly shortened.

  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 test part, a constant temperature testing room at 25°C can be established.

5.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.