Derating Lithium-Ion Batteries

Derating is a common practice for ensuring the longevity and safety of electronic components. It involves operating a device below its maximum rated capability to mitigate stress and extend its operational life. While derating guidelines are well-established for many electronic components, there's a notable gap when it comes to lithium-ion batteries. This blog post explores the concept of derating specifically for lithium-ion batteries, aiming to shed light on its importance and provide practical insights for those involved in battery applications.

Why Derate Li-ion Batteries?

Lithium-ion batteries, like all engineered systems, degrade over time, leading to capacity loss. This degradation is exacerbated by various stress factors encountered during storage and cycling. Derating helps to mitigate these stresses, ultimately extending the battery's calendar and cycle life.

 

What Factors Influence Derating?

Numerous stress factors can impact a lithium-ion battery's lifespan.

 

For Calendar Life:

• Temperature: Storage temperature is the most critical factor impacting calendar life. Elevated temperatures accelerate degradation processes, including SEI layer growth. Derating the temperature, ideally to room temperature, effectively minimizes these effects.
• State of Charge (SOC): Storing batteries at a high SOC can accelerate degradation. Derating the SOC, particularly at high temperatures, can help preserve calendar life.

For Cycle Life:

• Temperature: Similar to calendar life, high operating temperatures are detrimental. Elevated temperatures speed up side reactions and stress the battery, leading to faster capacity fade.
• Discharge C-rate: The rate at which a battery is discharged can impact its life. High discharge rates can lead to increased stress, potentially shortening cycle life. However, derating the discharge C-rate doesn't always guarantee prolonged life and should be evaluated on a case-by-case basis.
• Charge C-rate: High charging rates generate heat and can accelerate degradation processes. Derating the charge C-rate, particularly at lower temperatures, can be beneficial.
• Charge Cut-off Current: This parameter determines the current at which the constant voltage charging phase ends. Derating the charge cut-off current, especially at high temperatures, can reduce the stress associated with prolonged charging at high voltages.
• Charge Cut-off Voltage: This factor directly influences the battery's maximum SOC. While a higher cut-off voltage means more energy stored, it also exposes the battery to higher stress levels, accelerating degradation.

How to Derate Effectively

Derating is not a one-size-fits-all solution and requires a nuanced approach. Here’s a suggested methodology:

• Identify Stress Factors: Determine the specific stress factors relevant to your battery application.
• Test and Analyze: Conduct thorough testing under various stress levels using a statistically sound design of experiments (DOE). This will enable accurate calculation of derating factors for each stress factor.
• Choose and Implement: Based on the DOE results, select the stress factors you will derate, considering both ease of controllability and the magnitude of their derating factor.

Important Considerations

• Battery Chemistry: While this article focuses on general principles, remember that the specific electrochemical characteristics of different battery chemistries (LCO, LFP, etc.) will influence their responses to derating.
• Manufacturer Variations: Battery designs and manufacturing processes can differ significantly between manufacturers. Always consult the manufacturer's datasheet and recommendations for your specific battery model.
• Trade-offs: Derating inevitably involves trade-offs. For instance, reducing the charge C-rate extends the battery's life but increases charging time. Balancing these trade-offs is crucial for optimal performance.

Future Directions

The field of derating for Li-ion batteries is ripe for further exploration. Some potential areas of research include:

• Dynamic Derating: Developing dynamic derating strategies that adjust to real-time operating conditions and battery state could further enhance longevity and performance.
• Economics of Derating: Quantifying the economic benefits of derating, considering extended lifetime and reduced replacement costs, is crucial for widespread adoption.
• Combined Approaches: Investigating the synergy between derating and other battery management techniques, such as advanced thermal management or active balancing, could unlock even greater performance and lifespan improvements.

Conclusion

Derating is a powerful tool for maximizing the lifespan and performance of lithium-ion batteries. By understanding the factors influencing degradation and implementing appropriate derating strategies, battery users can ensure their systems operate reliably and efficiently for extended periods. As we transition to a future increasingly reliant on battery technology, a deep understanding and skillful implementation of derating will be crucial for success.