About the Capacity Fade
Capacity fade is the gradual loss of a battery's usable energy storage over its lifetime due to electrochemical degradation. It reduces both the energy that can be stored per cycle and the peak power the battery can deliver — directly affecting backup time and revenue in storage projects.
Formula
Capacity at cycle n = C₀ × (1 − fade_rate × n)
- C₀
- Initial nameplate capacity (kWh)
- fade_rate
- Capacity loss per cycle (e.g. 0.005% per cycle for LFP = 20% loss over 4,000 cycles)
- n
- Number of completed cycles
How to calculate it
- 1Obtain the manufacturer's cycle-life curve showing capacity vs. cycle count at the operating DoD.
- 2Fit the curve to a linear or exponential model for the operational range (typically 0–80% loss).
- 3Project capacity at each year of the project: capacity at year t = C₀ × capacity_fraction(annual_cycles × t).
- 4Recalculate backup time and energy throughput at each year using the degraded capacity.
- 5In revenue models, account for falling throughput — a 10-year BESS in year 8 delivers only 85–90% of its Year-1 energy per cycle.
Worked example
A 200 kWh LFP battery fading at 0.004% per cycle at 80% DoD. After 3,650 cycles (10 years at 1 cycle/day): capacity = 200 × (1 − 0.00004 × 3,650) = 200 × 0.854 = 170.8 kWh. Usable at 90% DoD, 93% RTE: 170.8 × 0.90 × 0.93 = 142.9 kWh — vs 167.4 kWh in Year 1.
What you can calculate
- Fade % per year
- NREL model
- LFP/NMC
- End-of-life capacity
Frequently asked questions
Can capacity fade be reversed or slowed?
Capacity fade cannot be reversed but can be slowed significantly by: (1) keeping the SoC window narrow (avoid fully charging or fully discharging), (2) maintaining the battery at 20–35°C with active cooling in hot climates, (3) limiting C-rate (charging/discharging slowly), and (4) preventing deep DoD cycles. BMS quality — how well it enforces these limits — is a primary determinant of real-world fade rate.
What happens when a BESS reaches end of life?
End-of-life (typically 80% of original capacity) means the battery can no longer meet the original design specs — backup time is 20% shorter, peak power falls, and LCOS rises. Options: (1) replace the battery pack (cells only, reusing the BMS and PCS reduces cost), (2) repurpose for a less demanding use case (second life at 60–70% capacity for stationary energy storage), or (3) recycle. India's battery recycling regulation (Battery Waste Management Rules 2022) requires EPR compliance.
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