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Battery Capacity & C-Rate Effects

About This Model

This interactive tool explores why not all of a battery's rated capacity is actually usable. You can:

  • Adjust the discharge rate (C-rate) and observe how it affects usable capacity
  • Change internal resistance to see how it causes voltage sag
  • Modify the cut-off voltage to understand when a battery is considered "empty"
  • Visualize the relationship between current, resistance, and voltage drop (V = I × R)

Key concepts: C-rate (discharge rate), internal resistance, voltage sag, cut-off voltage, and the difference between rated capacity and usable capacity. The shaded area under the discharge curve represents the energy delivered (voltage × capacity). Understanding these principles is essential for batteries in electric vehicles, portable electronics, and energy storage systems.

Discharge Controls

1.0 C
Current: 2.00 A
0.15 Ω
Voltage drop: 0.30 V
3.00 V
Depth of Discharge: ~100%

Results

Rated Capacity: 2.00 Ah

Representative capacity for a typical single-cell Li-ion battery (e.g., smartphone battery)

Usable Capacity: --- Ah
Accessible: ---
Energy Delivered: --- Wh

Note: The "lost" capacity is still in the battery but inaccessible at this discharge rate and cut-off voltage.

Voltage Range: Fully charged battery starts at 4.2V. Discharge stops at cut-off voltage (4.2V → 3.0V, using ΔV = 1.2V).

Understanding C-rate, Internal Resistance, and Depth of Discharge

C-rate describes how fast a battery discharges relative to its capacity. 1C = discharge in 1 hour. 2C = discharge in 30 minutes. 0.5C = discharge in 2 hours.

Formula: Current (A) = C-rate × Capacity (Ah)

Higher C-rate → Higher current → Greater voltage sag due to internal resistance (Vdrop = I × R). The battery hits the cut-off voltage sooner, leaving charge "stranded" inside.

Note: The shaded area under the discharge curve represents the energy delivered (voltage × capacity = Wh).

Cut-off voltage determines depth of discharge (DoD). Lower cut-off = deeper discharge = more capacity extracted, but may reduce battery lifetime. Higher cut-off = shallower discharge = preserves battery health.

Typical Li-ion: 3.0V (100% DoD) gives maximum capacity; 3.5V (~60% DoD) balances capacity with longevity.