🔌 Battery Voltage Behavior and Internal Resistance

Understanding voltage behavior during discharge is essential for evaluating a battery’s power performance. A simple yet effective approach is to model the battery using an equivalent circuit composed of:

• An open-circuit voltage (OCV) source
• A single internal resistance: R0

This model captures key dynamics such as voltage drop under load.

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⚙️ Equivalent Circuit Model

Figure: Simple Battery Equivalent Circuit

The terminal voltage ( V ) during discharge can be expressed as:

$$ V = \text{OCV} - I \cdot R_{\text{0}} $$

Where:

• OCV(SOC): Open-circuit voltage as a function of state of charge

• I: Discharge current (positive when discharging)

• R0: Internal resistance (ohms)

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📉 Voltage vs. Depth of Discharge

Figure: Discharge voltage curves at different C-rates

Observations:

• OCV Curve: Represents the equilibrium voltage without current.
• Voltage Drop: Increases with higher current due to I*R0 .
• Higher C-Rate → Lower Terminal Voltage: At 1.5C, the voltage drops more rapidly compared to 0.5C.

This behavior illustrates the importance of accounting for internal resistance in both modeling and real-world performance evaluations.

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