Enter your battery's electromotive force (EMF), terminal voltage, and load current to calculate the internal resistance of a battery. You'll get the internal resistance (r), total resistance (R_T), and a breakdown of how voltage is distributed across the internal and external resistances.
Internal Resistance (r)
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Load Resistance (R)
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Total Resistance (R_T)
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Voltage Drop (Internal)
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Power Lost (Internal)
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Internal resistance (r) is calculated using the formula r = (EMF − Terminal Voltage) / Current, or r = (ε − V) / I. The difference between the EMF and the terminal voltage represents the voltage dropped across the internal resistance, and dividing by the current gives the resistance value in ohms.
EMF (electromotive force) is the open-circuit voltage a battery can produce with no current flowing. Terminal voltage is the voltage measured at the battery's terminals when current is being drawn. The difference between the two is caused by the voltage drop across the battery's internal resistance.
Terminal voltage is calculated as V = ε − I × r, where ε is the EMF, I is the current, and r is the internal resistance. As the current drawn increases, the terminal voltage drops further below the EMF.
An ideal voltage source has zero internal resistance. This means its terminal voltage always equals its EMF regardless of the current drawn. Real-world batteries always have some internal resistance, which causes a voltage drop under load.
An ideal current source has infinite internal resistance. This ensures the same current flows regardless of the load connected to it. In practice, real current sources have very high but finite internal resistance.
Internal resistance determines how much power is lost as heat inside the battery and limits the maximum current it can deliver. A battery with low internal resistance (like a truck battery) can deliver high currents with minimal voltage drop, while a high internal resistance battery (like a motorcycle battery) drops significantly under heavy load.
Total resistance (R_T) is the sum of the internal resistance (r) and the external load resistance (R), expressed as R_T = r + R. It determines the total current that flows when the battery is connected to a circuit, according to Ohm's law: I = ε / R_T.
Temperature significantly affects internal resistance. Cold temperatures increase internal resistance, which is why car batteries struggle in winter — the higher resistance reduces the current available to start the engine. Higher temperatures generally lower internal resistance but can accelerate battery degradation over time.