As the core power source of electric vehicles, energy storage systems and various portable electronic devices, the discharge capacity of lithium battery PACK directly determines the endurance and use efficiency of the equipment. Discharge capacity is one of the important indexes to measure the performance of lithium batteries, which refers to the total amount of electricity that can be output by batteries under specific conditions. However, in practical applications, the discharge capacity of lithium battery PACK is often affected by many factors, resulting in unsatisfactory capacity performance. This article will deeply discuss the main factors that affect the discharge capacity of lithium battery PACK to help understand the causes of battery performance fluctuation.

I. Battery monomer quality and consistency

lithium battery PACK is composed of multiple battery cells in series and parallel. The performance of each monomer directly affects the Capacity Performance of the entire PACK. The quality of battery monomer includes its raw materials, manufacturing process and internal structure design, etc:

• capacity difference: the capacity of different units varies, which will lead to the reduction of the overall PACK capacity. The monomer with lower capacity becomes the bottleneck, limiting the play of discharge capacity.
• Internal resistance difference: The battery monomer with high internal resistance will produce large heat and voltage loss when discharging, which will affect the output efficiency of the whole PACK.
• Uneven Aging: during use, some monomer may accelerate aging due to different environment or use conditions, and the capacity decay is faster, affecting the overall capacity.

Therefore, to ensure the quality consistency of battery cells is to improve the PA of lithium battery.The basis of CK discharge capacity.

II. Assembly process and electrical connection quality

the assembly process and electrical connection status of lithium battery PACK have a significant impact on the discharge capacity:

• welding quality: if virtual welding, cold welding or broken welding occurs in the solder joints between battery cells, the contact resistance will be increased, resulting in voltage drop and capacity loss.
• Connection method: reasonably design the series-parallel structure to avoid voltage imbalance caused by excessive series connection, ensure the current sharing of parallel monomer, and reduce the capacity reduction caused by uneven current distribution.
• Mechanical stress: the mechanical pressure applied during the assembly process may damage the battery monomer or connecting parts, affecting the stability of the internal structure of the battery, thus reducing the capacity.

High quality assembly process and stable electrical connection are important factors to ensure the normal discharge capacity of lithium battery PACK.

III. Performance of battery management system (BMS)

the battery management system monitors the voltage, current, temperature and state balance of the battery in real time, which is the key guarantee for the safety and performance of the lithium battery PACK. The impact of BMS on discharge capacity is mainly reflected in:

If the function of BMS is not perfect or invalid, it may lead to unbalanced discharge of the battery, insufficient utilization of the capacity, and even safety accidents.

IV. Influence of temperature on discharge capacity

temperature is an important environmental factor affecting lithium battery performance, especially discharge capacity:

• low temperature environment: low temperature will increase the internal resistance of the battery and slow down the movement speed of electrolyte ions, resulting in a significant decrease in the discharge capacity. The available capacity of the battery under cold conditions is usually only 70%-80% of that at normal temperature.
• High temperature environment: although high temperature may improve the output capacity of the battery in a short period of time, long-term high temperature will accelerate the aging of the battery, resulting in faster capacity attenuation.
• Temperature uniformity: in the battery pack, uneven temperature will lead to overheating or supercooling of some battery cells, affecting their capacity and life performance.

Therefore, a reasonable thermal management system is the key to ensure that the lithium battery PACK maintains stable discharge capacity under different temperature environments.

V. Discharge rate (C rate)

discharge rate refers to the ratio of battery discharge current to its rated capacity, which directly affects the discharge capacity performance:

• high rate discharge: When the battery is discharged at a high rate, the internal resistance of the battery leads to a significant decrease in voltage, insufficient energy release of the battery, and the actual discharge capacity is lower than the rated value.
• Low rate discharge: when discharging at low rate, the battery can fully release the stored electric energy, and the discharge capacity is close to the rated capacity.

Different application scenarios have different discharge rate requirements, and the design needs to fully consider to balance the capacity and power requirements.

6. Cycle times and battery aging

the discharge capacity of lithium battery gradually decreases with the increase of the number of charge and discharge cycles. The reasons include:

active substance loss: the electrode material gradually loses its activity during charging and discharging, resulting in a decrease in capacity.
• Electrolyte decomposition: The long-term use of electrolyte will produce by-products, which will hinder ion migration and reduce battery efficiency.
• Structural change: structural changes of electrode materials and irreversible loss of lithium ions both lead to capacity loss.

The capacity of the aging battery PACK is significantly reduced, and scientific management of battery life is needed in practical applications.

VII. Environmental humidity and storage conditions

although humidity has relatively little effect on the discharge capacity of lithium batteries, poor storage environment still indirectly affects the battery performance:

• high humidity environment: it may cause corrosion of the battery shell or moisture of the electrolyte, affecting the stability of the internal structure of the battery.
• Storage power status: When the battery is stored for a long time, the state of charge (SOC) affects the capacity retention, and unreasonable storage power will accelerate the capacity attenuation.

Standardized storage management helps delay lithium the attenuation of capacity.