Understanding the longevity of lithium-ion batteries is crucial, especially as they become integral to various applications, from electric vehicles to renewable energy storage. Two primary metrics define battery lifespan: cycle life and calendar life. While they both pertain to battery degradation, they represent different aspects of performance decay.

Cycle Life: Measuring Usage-Based Degradation

Cycle life refers to the number of complete charge and discharge cycles a battery can undergo before its capacity falls to a specified percentage (commonly 80%) of its original capacity. This metric is pivotal in assessing the battery’s durability under regular use.

Factors Influencing Cycle Life:

• Depth of Discharge (DoD): Deeper discharges can shorten cycle life. For instance, discharging a battery to 100% DoD may result in fewer cycles compared to 50% DoD.
• Charge/Discharge Rates: High rates can generate more heat, accelerating degradation.
• Operating Temperature: Elevated temperatures can speed up chemical reactions, leading to faster capacity loss.
• Battery Chemistry: Different chemistries exhibit varying cycle lives. For example, Lithium Iron Phosphate (LFP) batteries often support more than 3,000 cycles under most conditions, and under optimal conditions, they support more than 10,000 cycles. Nickel Manganese Cobalt (NMC) batteries typically offer about 1,000 to 2,300 cycles, depending on conditions.

Illustrative Data:

• At 25°C with 100% DoD:
  • After 6,000 cycles: ≥80% capacity retention.
  • After 9,000 cycles: ≥70% capacity retention.
  • After 12,000 cycles: ≥60% capacity retention.
• At 45°C with 100% DoD:
  • After 3,000 cycles: ≥80% capacity retention.
  • After 4,500 cycles: ≥70% capacity retention.
  • After 6,000 cycles: ≥60% capacity retention.

Calendar Life: Time-Induced Degradation

Calendar life denotes the duration a battery can retain its performance while not in active use. Even when idle, batteries undergo chemical reactions that degrade their components over time.

Factors Influencing Calendar Life:

• Storage Temperature: Higher temperatures accelerate chemical degradation.
• State of Charge (SoC): Storing batteries at high SoC levels can lead to faster capacity loss.
• Battery Chemistry: Some chemistries are more stable over time. For instance, LFP batteries often have a longer calendar life than nickel-rich chemistries.

Typical Calendar Life Expectations for LFP Batteries:

• 15 years: ≥80% capacity retention.
• 18 years: ≥70% capacity retention.
• 20 years: ≥60% capacity retention.

These figures assume optimal storage conditions, such as moderate temperatures and SoC levels.

Balancing Cycle and Calendar Life

While cycle life focuses on usage-induced degradation, calendar life emphasizes time-induced degradation. In real-world applications, both factors interplay. For instance, a battery used infrequently but stored at high temperatures may experience significant capacity loss due to calendar aging.

Strategies to Maximize Battery Lifespan:

• Temperature Management: Store and operate batteries in environments with moderate temperatures.
• Optimal SoC Storage: Maintain batteries at a partial charge (typically around 40-60%) during storage.
• Regular Usage: Periodic cycling can help maintain battery health, preventing certain degradation mechanisms.
• Advanced Battery Management Systems (BMS): Implement BMS that monitor and adjust charging patterns, temperatures, and other parameters to optimize battery health.

Understanding the distinctions between cycle life and calendar life is essential for optimizing battery usage, ensuring longevity, and maximizing return on investment. By considering both metrics and implementing appropriate strategies, users can enhance the performance and durability of their lithium-ion batteries.