What Is Calendar Aging?

Calendar aging refers to the deterioration of battery components due to chemical reactions that occur over time, independent of charge-discharge cycles.

Calendar aging significantly impacts Battery Energy Storage Systems (BESS) by causing capacity degradation and increased internal resistance over time, even when the batteries are not actively cycled. This degradation affects the performance, reliability, and economic viability of BESS installations.

Key factors influencing calendar aging include:

• Temperature: Elevated temperatures accelerate chemical reactions within the battery, leading to faster degradation.
• State of Charge (SoC): Storing batteries at high SoC levels (e.g., above 80%) can increase the rate of capacity loss.
• Time: Prolonged storage, even under optimal conditions, contributes to gradual degradation.

These factors lead to the growth of the Solid Electrolyte Interphase (SEI) layer on the anode, consuming active lithium and increasing internal resistance, thereby reducing the battery’s capacity and efficiency.

Impact on Battery Energy Storage Systems (BESS)

1. Capacity Reduction: Calendar aging diminishes the energy storage capacity of BESS, limiting the amount of energy available for dispatch and reducing the system’s effectiveness in applications like peak shaving or load balancing.
2. Increased Internal Resistance: As internal resistance rises, the efficiency of energy conversion decreases, leading to higher energy losses during charging and discharging processes.
3. Economic Implications: Decreased capacity and efficiency can shorten the service life of BESS, impacting return on investment and increasing the levelized cost of storage.
4. Operational Challenges: To mitigate the effects of calendar aging, BESS operators may need to implement more sophisticated energy management strategies, potentially increasing operational complexity and costs.

Mitigation Strategies

To minimize the impact of calendar aging on BESS:

• Temperature Control: Maintain optimal operating temperatures (typically around 25°C) to slow down degradation processes.
• SoC Management: Avoid storing batteries at high SoC levels for extended periods; instead, maintain a moderate SoC (e.g., 40-60%) during idle times.
• Regular Cycling: Periodic charging and discharging can help prevent the buildup of detrimental chemical layers within the battery.
• Advanced Battery Management Systems (BMS): Utilize BMS with algorithms designed to optimize charging patterns and thermal conditions, thereby extending battery life.

Understanding and addressing calendar aging is crucial for the sustainable operation of BESS. By implementing appropriate mitigation strategies, operators can enhance system longevity and performance.