The charge and discharge criteria of constant voltage, constant current charging and constant discharge for batteries will be discussed. And this article is able to analyze the influence of distribution consistency, charging mode, discharge rate and ambient temperature on the battery discharge capacity.
1. Introduction of the battery discharge capacity
Lithium-ion batteries have the advantages of large capacity, high specific energy, good cycle life, no memory effect, etc., and have developed rapidly. Capacity, as its most critical performance index, has also attracted the attention of top 10 lithium battery companies in the world.
Correspondingly, lithium batteries are constantly developing in the direction of large capacity, fast charging, long life, and high safety, and new requirements are put forward for process technology in their manufacturing process.
Lithium ion battery pack is mainly used to test the electrical properties of battery cells after screening, matching, packaging, and assembly and to determine whether the difference in capacity and voltage is qualified.
2. Effect of matching consistency on battery discharge capacity
Good matching can not only improve the utilization rate of the cell, but also control the consistency of the monomer, which is the basis for achieving good battery discharge capacity and cycle stability for battery pack discharge.
However, the degree of AC impedance discretization of poorly matched battery cell capacity will increase, which in turn weakens battery pack cycle performance and available battery discharge capacity.
The difference between individual cells is used to evaluate the method. Realize the comprehensive evaluation and comparison of battery performance, and transform the qualitative analysis of battery performance into quantitative analysis.
The performance evaluation of the power supply batteries was performed. The dynamic characteristic matching method mainly realizes the matching function according to the battery charge and discharge curves.
The specific implementation step is to first extract the feature points on the curve, realize the classification of the curve by selecting a suitable algorithm, and then complete the matching process of the battery.
This pairing method takes into account changes in battery performance during operation. On this basis, other suitable parameters are selected for battery matching, and batteries with more consistent performance can be separated.
3. How charging method effect battery discharge capacity
The appropriate charging system has an important impact on the battery discharge capacity.
If the charging depth is shallow, the battery discharge capacity will be reduced accordingly. If overcharged, it will affect the battery's chemically active substances, causing irreversible damage and reducing the battery's capacity and life.
Therefore, it is necessary to select the appropriate charging ratio, upper limit voltage, and constant voltage cut-off current to ensure that the charging efficiency and safety stability are optimized while the charging capacity is achieved. At present, lithium-ion batteries mostly adopt the constant current-constant voltage charging mode.
Analysis of the constant current and constant voltage charging results of lithium iron phosphate system and ternary system batteries under different charging currents and different cut-off voltages shows that:
- When the charging voltage is set, the charging current increases, the constant current ratio decreases, the charging time is shortened, but the energy consumption increases.
- When the charging current is fixed, with the decrease of the charging off voltage, the constant current charging ratio decreases, charging capacity and energy decrease. In order to ensure the battery discharge capacity, the charging cutoff voltage of the lifepo4 batteries cannot be lower than 3.4 V.
It is necessary to balance charging time and energy loss, and select the appropriate charging current and shutdown time. The consistency of SOC in each cell largely determines the battery discharge capacity pack, and the equalization charge provides the possibility to realize the similarity of the initial SOC platform of each cell discharge, which can improve the battery discharge capacity and efficiency.
The charging equalization method refers to the equalization of the power battery in the charging process, generally starting when the cell voltage of the battery pack reaches or exceeds the set voltage, and preventing overcharging by reducing the charging current.
4. How discharge rate influence battery discharge capacity
The discharge rate is a crucial indicator for power batteries. The large rate discharge of the battery is a test for cathode and anode materials and electrolytes. For lithium iron phosphate cathode material, its structure is stable, the strain in charge and discharge processes are small, and it has the basic conditions for high-current discharge.
However, the disadvantage is that lithium iron phosphate has poor conductivity. The diffusion rate of lithium ions in the electrolyte is the main factor affecting the discharge rate of the battery, and the diffusion of ions inside the battery is closely related to the structure and electrolyte concentration of the battery.
Therefore, different discharge rates lead to different discharge times and discharge voltage platforms of the battery, which in turn lead to different battery discharge capacities, which is particularly obvious for parallel battery packs. It is necessary to choose the appropriate discharge rate.
The battery's usable capacity decreases as the discharge current increases. When lithium-ion batteries are discharged, the standard 1C is generally selected, and the maximum discharge current is usually limited to 2-3C.
When discharged with a large current, a large temperature rise will be generated, and energy loss will be generated. Therefore, it is necessary to monitor the battery temperature in real time to prevent damage to the battery caused by excessive temperature and reduce battery discharge capacity.
5. The influence of temperature on battery discharge capacity
Temperature mainly affects the activity of the electrode material and electrolyte performance inside the battery. Too high or too low temperatures have an effect on the battery discharge capacity.
● Low temperature
At low temperatures, the battery's activity is significantly reduced, lithium's ability to be embedded and taken out decreases, the battery's internal resistance and polarization voltage increase, and the actual usable 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, which is manifested as a decrease in the usable capacity of the battery and a decrease in the energy utilization efficiency of the battery.
● High temperature
When the temperature rises, the lithium ion detachment and embedding between the cathode and anode becomes active, so that the internal resistance of the battery is reduced, the internal resistance stability time is longer, and the capacity is more effectively exerted.
However, if the battery operates in a high temperature environment for a long time, the stability of the cathode lattice structure will deteriorate, the safety of the battery will be reduced, and the battery life will be significantly shortened.
The analysis considers that the change rate of ohmic internal resistance at low temperature is greater than the change rate at high temperature, indicating that low temperature has a more obvious effect on battery activity, thus affecting battery discharge capacity.
As the temperature rises, the ohmic internal resistance and polarization internal resistance of the charging and discharging processes decrease. However, at higher temperatures, it will destroy the chemical reaction balance and material stability in the battery, resulting in possible side reactions.
This affects the battery discharge capacity and internal resistance of the battery, resulting in a shortened cycle life or even reduced safety. Therefore, high and low temperatures will affect the performance and service life of lithium iron phosphate batteries, and the actual working process should be ensured by increasing battery thermal management and other methods to ensure that the battery operates under suitable temperature conditions.
The consistency between battery series and parallel cells is in the battery need to be specially considered. If the consistency is poor will seriously affect the overall performance of the battery, and even cause overcharge or overdischarge to cause potential safety hazards.
Batteries are subject to the influence of ambient temperature, and too high or too low temperature will have an impact on battery capacity. Due to the polarization effect and thermal effect of the battery, the capacity will decrease, so it is necessary to choose the appropriate charge-discharge ratio.