Charging Profile (CC-CV)

How Does CC-CV Charging Work?

Constant Current – Constant Voltage (CC-CV) is the standard charging protocol for lithium-ion batteries. It consists of two phases: first, the charger applies a constant current until the cell reaches its maximum voltage (typically 4.2V for NMC/NCA), then it holds that voltage while the current gradually tapers down.

During the CC phase, the battery charges rapidly and absorbs roughly 60-80% of its capacity. During the CV phase, charging slows as the voltage difference between charger and cell decreases, delivering the remaining 20-40%. The CV cutoff current (typically C/20 to C/10) determines when charging is considered complete.

This protocol balances charging speed with battery longevity. Skipping or shortening the CV phase (charging only to ~80% SOC) significantly extends cycle life, which is why many EVs and laptops offer an '80% charge limit' setting.

The CV cutoff current setting directly affects both charge completeness and charging time. A lower cutoff (e.g., C/20) charges the cell to ~99% SOC but extends the CV phase significantly. A higher cutoff (e.g., C/5) ends charging at ~90-95% SOC but saves 30-60 minutes. Choosing the right cutoff is an engineering trade-off between usable capacity and total charge time.

Formula: CC Phase Time ≈ (Target SOC - Initial SOC) × Capacity / Current CV Phase Time ≈ -τ × ln(I_cutoff / I_cc) Total Time = CC Phase + CV Phase

Example Calculation

Charging a 5 Ah cell at 2.5A (0.5C) with 0.25A cutoff. CC phase delivers ~80% = 4 Ah at 2.5A → ~96 min. CV phase delivers ~1 Ah with tapering current → ~45 min. Total ≈ 141 min (2.35 hours) for a full charge.

When to Use This Calculator

• Designing a charger by estimating the total charge time for a given cell capacity, charge current, and cutoff current
• Optimizing charging parameters to balance charge speed vs. cycle life — comparing different CC rates and CV cutoff currents
• Estimating how much of the charge time is spent in the CC phase vs. the CV phase to inform user-facing charge time predictions
• Planning test station throughput in manufacturing by calculating per-cell charge time for formation cycling

Common Mistakes to Avoid

• Setting the CV cutoff current too high — ending at C/5 instead of C/20 leaves 5-10% of capacity uncharged, which compounds across parallel cells in a pack causing imbalance
• Ignoring temperature limits during CC phase — high CC current generates significant heat; if the cell exceeds 45°C, the BMS should derate or pause charging to prevent accelerated degradation
• Applying CC-CV parameters from one chemistry to another — NMC charges to 4.2V, LFP to 3.65V, LTO to 2.85V; using the wrong voltage ceiling is dangerous
• Not accounting for the CC phase SOC starting point — if the cell is not fully empty, the CC phase is shorter; the model assumes starting from 0% unless adjusted

Frequently Asked Questions

Why is constant voltage needed — why not just constant current?

Constant current alone would push the cell voltage above safe limits as SOC increases, causing lithium plating, electrolyte decomposition, and potentially thermal runaway. The CV phase limits voltage while allowing current to naturally decrease as the cell approaches full charge.

What is the difference between CC-CV and step charging?

CC-CV uses one current level then voltage hold. Step charging (multi-stage CC) uses decreasing current steps without a CV hold, which can reduce charging time while limiting heat generation. Some advanced protocols like pulse charging add rest periods to allow lithium-ion diffusion, potentially improving cycle life.

How do I choose the right CC charging current?

Refer to the cell manufacturer's maximum recommended charge rate (e.g., 0.5C or 1C). Higher CC current shortens the CC phase but generates more heat and stress. For cells without active thermal management, 0.5C is a safe default. For fast-charging applications, high-power cells rated for 2-4C charging are required.