Internal Resistance Calculator

Your details

Choose which quantity you want the calculator to compute. All others become required inputs.
Electromotive force - the open-circuit voltage a battery or generator can deliver with no current flowing.
V
External resistance connected to the battery terminals. The calculator uses this to compute current if needed.
ohm
Current flowing through the circuit when the load is connected.
A
Internal Resistance (r)Good efficiency
2ohm

Resistance inside the battery or source opposing current flow

Terminal Voltage (V)10V
EMF12V
Current (I)1A
Internal Voltage Drop2V
Internal Power Loss2W
Power Delivered to Load10W
Efficiency83.33%
Total Circuit Resistance12ohm
83.33% %
Poor<60%Moderate60%-80%Good80%-95%Excellent95%+
Load Power (W)10
Internal Loss (W)2

Internal resistance is 2.0000 ohm with 83.3% efficiency.

  • The source delivers 83.3% of its power to the external load; the remaining 16.7% is dissipated as heat inside the source.
  • The internal voltage drop is 2.000 V, so the terminal voltage under this load is 10.000 V compared with an open-circuit EMF of 12.000 V.
  • At 1.000 A, internal heating is 2.000 W. Drawing more current will increase this quadratically.

Next stepTo measure internal resistance experimentally, record terminal voltage at two different load currents and use the two-point method: r = (V1 - V2) / (I2 - I1).

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