Calculate source or battery internal resistance, loaded voltage, or current from EMF and voltage sag, with milliohms, heat loss, efficiency, and a planned-load check.

Linear DC discharge model at one operating condition. Do not short a real source; model limits are not safe current ratings.

Optional settings

Blank omits the second operating-point estimate.

Internal Resistance Formula

The following formula is used to calculate an internal resistance.

r = (e / i) - R
  • Where r is the internal resistance (ohms)
  • e is the electromotive force (volts)
  • i is the current (amps)
  • R is the resistance of the load in the circuit.

To use the load-resistance mode, divide the electromotive force by the current, then subtract the external load resistance. Equivalently, with measured terminal voltage V, use r = (e – V) / i. The model describes DC discharge with a nonnegative terminal voltage; it does not predict a safe source current rating.

Internal Resistance Definition

Internal resistance is the effective resistance inside a source of electromotive force (such as a battery, generator, or power supply) that causes the terminal voltage to drop when current flows.

Internal Resistance Example

How to calculate the internal resistance?

  1. First, determine the electromotive force.

    For this example we will say this value is 10 V.

  2. Next, determine the current.

    For this example we will assume a current of 0.4 amps.

  3. Next, determine the load resistance.

    For this example the value is 20 ohms.

  4. Finally, calculate the internal resistance.

    Using the formula we find the internal resistance to be 10/0.4 – 20 = 5 ohms.