The key discharge-curve analysis begins with three voltage regions: the exponential area, the plateau area, and the cutoff area. For newly fabricated cells, characterize the initial voltage drop, the stable operating plateau, and the final polarization-driven decline. Record Vfull, Vexp, and Vnom, then interpret them alongside capacity, internal resistance, current rate, temperature, and cutoff limits.
A discharge curve is not only a capacity measurement. Its regions and voltage landmarks reveal how the cell reacts at the beginning of discharge, during stable energy delivery, and near depletion, providing the inputs needed for performance comparison and equivalent-circuit modeling.
Identify the Main Discharge-Curve Regions
The Exponential Area
The exponential area is the initial portion of constant-current discharge, where terminal voltage drops rapidly from the fully charged condition.
This region reflects the cell’s immediate electrochemical and ohmic response. Its shape can reveal differences in electrode formulation, active-material utilization, polarization, and initial internal resistance.
The Plateau Area
The plateau area follows the initial voltage drop and represents a relatively stable electrochemical reaction equilibrium.
Voltage declines slowly across this region while the cell delivers most of its usable energy. Important measurements include the plateau voltage, its duration, and the capacity delivered before the cell leaves this stable operating range.
The Cutoff Area
The cutoff area is the final portion of discharge, where voltage falls rapidly as polarization increases and the cell approaches its discharge limit.
The cutoff region should be analyzed carefully because a low terminal voltage does not necessarily mean that all active material has been reversibly used. Excessive polarization, high current, temperature, or resistance growth can cause the cell to reach the cutoff threshold prematurely.
Record the Essential Voltage Parameters
Vfull: Initial Full-Charge Voltage
Vfull is the initial terminal voltage measured when the cell begins discharge from its fully charged state.
It establishes the starting boundary of the discharge curve and provides a reference for comparing cells fabricated with different materials, formation procedures, or charging conditions.
Vexp: End of the Exponential Region
Vexp is the terminal voltage marking the transition from the initial rapid voltage decline to the more stable plateau.
This parameter helps quantify the magnitude of the early voltage loss. A large difference between Vfull and Vexp may indicate substantial initial polarization or internal resistance under the selected discharge current.
Vnom: End of the Plateau Region
Vnom is the finishing voltage of the plateau region, where the cell begins entering the rapid final voltage decline.
It is useful for estimating the practical operating voltage and for separating stable energy delivery from the end-of-discharge behavior. The relationship between Vnom and the formal cutoff voltage also indicates how much of the measured capacity is delivered before strong polarization dominates.
Cutoff Voltage
The cutoff voltage is the lower voltage limit at which the test terminates.
It must be defined with the discharge current, temperature, cell chemistry, and test standard because terminal voltage is load-dependent. Forcing discharge below an appropriate cutoff can damage the cell or distort comparisons between samples.
Connect Voltage Behavior to Cell Performance
Analyze Capacity Within Each Region
Rated capacity alone does not show where the cell delivers its energy.
Measure how much capacity is delivered in the exponential, plateau, and cutoff regions. A cell with a long, stable plateau generally provides more useful operating capacity than one whose apparent capacity is concentrated near the cutoff region.
Compare Different Current Rates
Discharge curves should be collected at more than one C-rate when the test objective includes power capability or rate performance.
Higher current generally increases voltage drop and polarization, shortens the usable plateau, and reduces effective capacity relative to nominal capacity. Comparing curves at controlled rates separates material limitations from behavior caused primarily by load.
Measure Internal Resistance
Internal DC resistance can be estimated from the voltage response to a current change:
[ R_{\mathrm{DC}} = \frac{\Delta V}{\Delta I} ]
Measure this response at defined state-of-charge points rather than relying only on a single resistance value. Resistance affects the gap between the cell’s open-circuit voltage and its loaded terminal voltage, particularly during the exponential and cutoff regions.
Include Temperature and State of Charge
Voltage parameters are meaningful only when the test conditions are documented.
Record temperature, discharge current, starting state of charge, rest period, and cutoff limit. Open-circuit-voltage-versus-state-of-charge mapping can provide additional context for distinguishing equilibrium voltage from voltage losses caused by current and polarization.
Use the Curve for Modeling and Material Evaluation
Build Equivalent-Circuit Models
The discharge curve, voltage landmarks, design capacity, and room-temperature internal resistance can be combined to establish an equivalent-circuit model.
Vfull, Vexp, and Vnom provide practical landmarks for representing the cell’s voltage behavior across different operating states. The model should also account for current rate and temperature if it will be used outside the original test conditions.
Evaluate Material Formulations
Changes in electrode density, active-material formulation, electrolyte composition, or current-collector and grid design can shift the curve’s shape.
Compare samples using consistent metrics: initial voltage loss, plateau voltage, plateau duration, delivered capacity, cutoff behavior, and resistance. This makes the discharge profile useful for diagnosing formulation changes rather than merely ranking total capacity.
Calculate Delivered Energy
Stored energy is obtained by integrating voltage over discharged capacity:
[ E = \int V,dQ ]
This matters because two cells with similar ampere-hour capacity can deliver different watt-hour energy if their average discharge voltages differ.
Extend the Profile Beyond One Discharge
A complete characterization program should supplement discharge curves with cycle-life, calendar-life, self-discharge, coulombic-efficiency, and impedance measurements.
These tests reveal whether the initial voltage profile remains stable or changes through capacity fade, resistance growth, side reactions, or storage-related degradation.
Understanding the Trade-offs
A Lower Cutoff Can Inflate Apparent Capacity
Extending discharge closer to, or below, the recommended cutoff may increase measured capacity while reducing practical relevance and potentially stressing the cell.
Use a chemistry- and manufacturer-appropriate cutoff, and apply the same limit to every comparison cell.
High-Rate Curves Are Not Directly Comparable to Low-Rate Curves
A high-rate discharge may show a lower plateau and an earlier cutoff because of increased ohmic loss and polarization.
This does not necessarily prove that the material has lower intrinsic capacity. Report the C-rate and distinguish rate-limited capacity from nominal-capacity performance.
A Single Voltage Curve Is Incomplete
One discharge curve cannot establish durability, safety, self-discharge, or frequency-dependent impedance.
Treat it as a baseline performance profile and combine it with controlled thermal, impedance, mechanical, and long-term cycling tests when the design decision requires broader evidence.
Chemistry Can Add More Detailed Regions
The three-region framework is a general constant-current interpretation. Some chemistries contain additional reaction-specific plateaus or transitions; for example, lithium-sulfur cells can show multiple electrochemical regions associated with polysulfide and lithium sulfide formation.
When such features appear, preserve the general landmarks but also annotate chemistry-specific transitions and plateau capacities.
How to Apply This to Your Project
Use a controlled constant-current test and report both the curve and the conditions that produced it.
- If your primary focus is baseline cell comparison: Measure Vfull, Vexp, Vnom, cutoff voltage, delivered capacity, plateau duration, and room-temperature internal resistance under identical conditions.
- If your primary focus is high-power performance: Repeat the analysis across relevant C-rates and temperatures, emphasizing voltage sag, DC resistance, usable capacity, and the onset of cutoff.
- If your primary focus is equivalent-circuit modeling: Combine the voltage landmarks with capacity, resistance, and open-circuit-voltage-versus-state-of-charge data collected at defined operating points.
- If your primary focus is material or formulation optimization: Compare the initial voltage drop, plateau stability, regional capacity, energy integration, and resistance growth across otherwise identical cells.
- If your primary focus is durability: Repeat discharge-curve measurements during cycle-life and calendar-life testing to track plateau shifts, cutoff behavior, capacity fade, and resistance increase.
A well-resolved discharge curve turns voltage changes into evidence about the cell’s electrochemical behavior, usable energy, and operating limits.
Summary Table:
| Region | Description | Key Parameters |
|---|---|---|
| Exponential | Initial rapid voltage drop from full charge; reflects ohmic and electrochemical response. | Vfull, Vexp |
| Plateau | Stable voltage region where most energy is delivered; voltage declines slowly. | Vnom, plateau voltage, delivered capacity |
| Cutoff | Final rapid decline as cell nears discharge limit; indicates polarization and resistance. | Cutoff voltage, capacity to cutoff |
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