How to Evaluate Lithium-Ion Battery Capacity Performance

Lithium-ion batteries are the backbone of today’s electronics, EVs, and energy storage systems. One of the most critical aspects of battery quality is capacity performance—how well a cell retains its discharge capacity over repeated charge and discharge cycles.

In this article, we outline the main testing methods for battery capacity evaluation, complete with key formulas and industry standards.

1. Cycle Life Testing

Cycle life testing is the core method used to assess lithium battery performance. It simulates real-world usage by repeatedly charging and discharging cells under controlled conditions.

Standard procedure includes:

  • Initial capacity test (C₁): Perform 1–3 charge/discharge cycles at 0.5C, and take the average discharge capacity as the baseline.

  • Charge/discharge protocol:

    • Charging: Constant Current–Constant Voltage (CC–CV), cutoff at 4.2V.

    • Discharging: Constant Current (CC), cutoff at 2.8V.

  • Cycle number (N): Typically 500–1000 cycles, depending on application.

  • Capacity checkpoints: Measure capacity every 50–100 cycles.

  • End-of-life threshold: When capacity falls to 80% of its initial value.

Key formulas:

Capacity Retention Rate (η):


η = (C_N / C_1) × 100%

Capacity Fade Rate (α):


α = 1 - η

Cycle Life:
The number of cycles until capacity retention first drops to 80%.

2. Accelerated Cycle Testing

To reduce testing time, accelerated cycle life testing exposes batteries to harsher conditions that speed up aging:

  • High temperature cycles (e.g., 45°C): Accelerates SEI growth and electrolyte decomposition.

  • High charge/discharge rates (≥1C): Increases polarization and mechanical stress.

  • Wider voltage windows (e.g., 2.5–4.3V): Amplifies electrode stress and degradation.

⚠️ Note: Overly aggressive testing may distort real-world failure mechanisms.

3. Calendar Life Testing

Calendar life focuses on capacity fade during storage, simulating long-term idle scenarios. Batteries are stored at fixed States of Charge (SOC) (commonly 50% or 100%) and monitored periodically.

Empirical capacity decay model:


C_t = C_0 - k × t

Temperature effect (Arrhenius equation):


k = A × e^(-Ea / RT)

Where Ea = activation energy, T = absolute temperature (K), R = gas constant.

4. Post-Mortem Analysis

After cycle testing, post-mortem analysis provides deeper insights into degradation mechanisms:

  • Electrode analysis: XRD/SEM to identify lattice collapse or graphite exfoliation.

  • Interface chemistry: XPS/FTIR to detect SEI/CEI composition changes.

  • Electrolyte analysis: GC-MS to measure decomposition products.

Conclusion

  • Cycle life testing is the gold standard for lithium battery capacity evaluation.

  • Accelerated testing saves time but must reflect real-world conditions.

  • Calendar life testing ensures reliability during long-term storage.

  • Post-mortem analysis uncovers root causes of capacity fade.

At YDL Battery, we not only manufacture custom lithium polymer and lithium-ion batteries, but also provide in-house testing and validation to ensure long cycle life and reliable performance for your applications.

👉 Contact us today to learn more about our custom battery solutions and technical support.