Specification sheets and battery cell lifetime is always about capacity loss, the percentage of the original capacity in Ah available versus number of cycles and time. However, the range of your vehicle and how long your battery can support your house for is dependent on the amount of energy available. Hence this paper caught our attention.
Abstract: Battery aging in research publications and manufacturer specification sheets for individual cells is commonly reported as capacity (Ah) versus cycle number. However, the key measured quantity in battery-powered devices is energy (Wh), which is derived from integrating capacity with voltage. In this work, we compare the rate of capacity and energy loss across a wide range of Li-ion single-cell cycling studies with different positive electrode chemistries, charge–discharge rates, and temperatures. We find that the relative rate of discharge energy loss varies with cycling conditions. For many cells cycled under moderate conditions, the rate of discharge energy fade is only slightly faster than the rate of discharge capacity fade. However, some cells demonstrated up to a 15% decline in cycle count when 80% energy retention rather than 80% capacity retention was used as the end-of-life metric. These results highlight the importance of reporting cell aging based on energy fade to avoid overestimating battery lifetime in full systems.
Preger, Y., Wittman, R., Harris, S. J., and Dubarry, M. (March 13, 2026). “Are Capacity and Energy Loss Equivalent Metrics for Battery Aging Reporting?.” ASME. J. Electrochem. En. Conv. Stor. May 2026; 23(2): 021109.
They evaluated the rate of discharge capacity and energy loss across many public Li-ion battery aging datasets. The datasets and coverage of different chemistries is interesting in itself.
The Y-axis of the plots shows relative rates of discharge energy and discharge capacity retention. A value below 100% indicates that the energy is fading faster than the capacity.
For nearly all cells, the discharge energy fades faster than the discharge capacity.
Why it matters: Any difference between the rate of capacity and energy fade will lead to inaccurate prediction of battery device performance from single-cell specs.
When we look at the DCIR of a Cell we see that the resistance increases with cycle number. This resistance also increases at low temperatures and hence discharging a high current from a cell that is cold and has been cycled a lot will result in a significant voltage drop. To the point that the minimum voltage is reached long before the cell is at minimum SOC.
Useful Life
Nominal capacity or energy retention reframed in terms of “useful” cell life.
At the same cycle, a cell in the UM Expansion dataset shows 20% discharge energy loss and 17% discharge capacity loss. The 3% difference seems negligible in the context of the overall nominal capacity or energy of the cell. However, when a battery’s end of useful life is considered 20% energy retention, this 3% difference corresponds to a 15% difference in useful life.
- Discharge energy almost always faded faster than discharge capacity due to changes in the mean voltage
- In the worst case, cells showed up to a 15% reduction in cycle count to EOL when using 80% energy retention rather than 80% capacity retention as the key metric
The last sentence in the paper abstract is a good place to conclude: These results highlight the importance of reporting cell aging based on energy fade to avoid overestimating battery lifetime in full systems.
References
- Preger, Y., Wittman, R., Harris, S. J., and Dubarry, M. (March 13, 2026). “Are Capacity and Energy Loss Equivalent Metrics for Battery Aging Reporting?.” ASME. J. Electrochem. En. Conv. Stor. May 2026; 23(2): 021109.
See also: Battery Capacity