Battery Cycle Life Capacity Fade

Formula

Description

Battery capacity degrades with each charge-discharge cycle due to irreversible chemical and mechanical changes in the electrode materials. The exponential fade model assumes a constant percentage loss per cycle. While simplified (real fade is not perfectly constant), it provides useful estimates for battery lifetime planning. Li-ion cells are typically considered end-of-life at 80% of initial capacity. The fade rate depends on chemistry, depth of discharge, charge rate, temperature, and voltage limits. LFP chemistry typically has lower fade rates than NMC or NCA.

Variables

• Qₙ — Remaining capacity after n cycles (Ah)
• Q₀ — Initial capacity (Ah)
• fade — Fractional capacity loss per cycle (e.g., 0.0002 for 0.02%/cycle)
• n — Number of charge-discharge cycles

Practical Notes

Example: Q₀ = 3000 mAh, fade = 0.0002/cycle, after 1000 cycles: Q = 3000 × (1 − 0.0002)^1000 ≈ 2456 mAh (81.9%). Solving for n at 80% capacity: n = ln(0.8)/ln(0.9998) ≈ 1116 cycles. Higher temperatures, deeper discharge, and faster charging all increase the fade rate. Calendar aging also contributes to capacity loss independent of cycling.