The longer a battery pack is used, the more its capacity generally declines, although the degradation rate varies among different products. Capacity degradation may lead to shorter runtime, reduced power supply capability, increased internal resistance, and higher maintenance costs. Understanding these causes can help extend battery life by improving materials, management methods, and usage practices. Studies show that battery aging is mainly related to the number of cycles, usage time, temperature, and operating conditions.
Aging of Cell Materials Causes Capacity Reduction
The capacity of a battery pack is based on its internal cells. When electrode materials and electrolytes change during long-term use, the amount of active material available for normal charging and discharging gradually decreases, causing the capacity to decline.
Gradual Degradation of Positive and Negative Electrode Materials
After a cell undergoes a large number of charge and discharge cycles, the positive and negative electrode materials continuously experience ion insertion and extraction. Long-term structural changes may result in the loss of active material.
- Changes in particle structure: Stress caused by repeated charging and discharging may lead to cracks or structural damage in some materials.
- Reduction in active sites: Some electrode materials gradually lose their ability to participate in electrochemical reactions.
- Decline in material utilization: As internal transport conditions within the electrodes change, the usable capacity may gradually decrease.
The long-term stability of electrode materials is an important foundation for maintaining battery pack capacity.
Side Reactions Between Electrodes and Electrolytes
Side reactions between electrodes and electrolytes consume some active material and may form interfacial layers that hinder ion transport, resulting in active material loss and increased interfacial impedance. Some interfacial films may continuously grow, rupture, and reform during cycling. Long-term side reactions may also gradually consume the electrolyte and reduce its effective participation. At the same time, increased interfacial impedance causes internal resistance to rise gradually, placing greater limitations on the output of the same amount of energy. As side reactions continue to accumulate, the usable capacity of the battery pack gradually decreases.
Charging and Discharging Conditions Accelerate Capacity Degradation
Excessive charge and discharge rates, overly deep cycling, and long-term operation within extreme SOC ranges may all increase the degradation stress on cells.
Excessively High Charge and Discharge Rates
High-current charging and discharging increase internal polarization and temperature rise within the battery. Long-term high-rate operation may accelerate material aging.
- High-rate charging: May increase the reaction stress at the electrode interface.
- High-rate discharging: Can easily generate significant internal heat.
- Frequent high-power operation: Causes the cells to withstand relatively high operating loads for extended periods.
Properly controlling charge and discharge rates helps reduce additional losses during battery pack operation.
Excessive Depth of Cycling
The capacity range used during each cycle also affects the long-term degradation rate.
- Frequent deep discharge: Causes the cells to experience a wider range of SOC changes.
- Long-term full charging and full discharging: May increase the operating stress on certain materials under extreme conditions.
- High-frequency cycling: Accumulates more charging and discharging stress within a short period.
Appropriately controlling the depth of cycling can reduce the long-term impact on cell structures.
Temperature and Environmental Factors Affect Capacity
Temperature is an important external factor affecting battery pack capacity degradation. High temperatures may accelerate side reactions, while low temperatures may reduce charging and discharging performance.
High Temperatures Accelerate Battery Aging
Higher temperatures promote electrolyte decomposition, interfacial reactions, and the aging of certain materials. Long-term exposure to high temperatures may shorten battery service life. When heat dissipation is insufficient, localized high-temperature areas may form inside the battery pack. High-temperature charging and discharging may further accelerate side reactions. Even when the battery is not operating, high-temperature storage may cause significant calendar aging. Effective thermal management helps maintain stable battery pack operation.
Low Temperatures Affect Charging and Discharging Conditions
In low-temperature environments, the rates of ion transport and electrochemical reactions inside the battery change, which may reduce charging and discharging capability.
- Low-temperature charging: The charging current must be controlled according to the characteristics of the cells.
- Low-temperature discharging: Usable capacity and output power may be affected.
- Repeated temperature changes: Differences in thermal expansion and contraction among different materials may increase structural stress.
Proper temperature management can reduce the adverse effects of extreme environments on battery performance.
Cell Consistency and Battery Management Affect Capacity
Battery pack capacity depends not only on the rated capacity of a single cell. When multiple cells operate together over the long term, consistency and BMS management also affect the actual usable capacity.
Gradual Increase in Differences Between Individual Cells
Cells inevitably have certain parameter differences during production and use, including capacity, internal resistance, and self-discharge level.
- Capacity deviation: Some cells may reach a fully charged or fully discharged state earlier than others.
- Differences in internal resistance: Cells with higher internal resistance are more likely to experience voltage fluctuations and heat generation.
- Different aging rates: After long-term cycling, performance differences among individual cells may become more pronounced.
As differences between individual cells increase, the overall usable capacity of the battery pack may be limited by the weaker cells.
Inadequate BMS Balancing and Protection
The BMS monitors the voltage, current, and temperature of the battery pack and improves operating conditions through protection and balancing strategies. When management strategies are inappropriate, differences between individual cells may further affect battery pack performance.
- Abnormal voltage monitoring: Changes in the condition of individual cells may not be detected in time.
- Insufficient balancing capability: The SOC gap between cells may continue to increase.
- Mismatched protection parameters: Cells may remain in unsuitable operating ranges for extended periods.
Appropriate BMS strategies can reduce the impact of cell differences on capacity retention.
In practical applications, the capacity status of the battery pack should be evaluated regularly based on operating data, so that abnormal cells and performance changes can be identified in a timely manner. Proper maintenance and safety management can also reduce the impact of capacity decline on runtime, power supply stability, and equipment service life. By establishing comprehensive monitoring, warning, and maintenance mechanisms, the performance degradation of the battery pack can be further delayed.
