Electrical Fundamentals
Source Internal Resistance Calculator
Source Internal Resistance provides a checkable estimate of internal resistance. It is intended for scenario comparison, not as automatic approval of a component or installation.
Enter values for Internal resistance
Do not mix values recorded under different conditions.
Compare two operating cases
Change Open-circuit voltage from 12.6 V to 15.12 V while preserving the other measurements. The output changes from 0.0400 Ω to 0.1660 Ω.
This isolates one variable; a real system may change several at once.
Internal resistance: calculation method
Here, Internal resistance is obtained from r = (Voc − Vload) ÷ I. Its variables correspond to Open-circuit voltage, Loaded voltage, and Load current.
With the current entries, the primary answer is 0.0400 Ω. Preserve the raw inputs when replacing this example.
Estimate source resistance from loaded voltage sag. When that related quantity matters, continue with Maximum Power Transfer Calculator.
Before entering values
Record conductor temperature and whether values are measured, nominal, or calculated. Do not combine a worst-case value with unrelated nominal data.
Values used below a division bar must stay positive. Check every prefix before entry; micro, milli, kilo, and mega are not interchangeable. If you also need internal resistance, continue with Battery Internal Resistance Calculator.
- Open-circuit voltage
- Default example: 12.6 V. Enter open-circuit voltage in V.
- Loaded voltage
- Default example: 11.8 V. Enter loaded voltage in V.
- Load current
- Default example: 20 A. Enter load current in A.
Applying the result
The calculated Internal resistance is meaningful only with its input conditions attached. Compare it with a second case representing tolerance or temperature.
Keep the input set beside the output so another reader can reproduce it. Pair this page with Battery Voltage Sag Calculator when reviewing loaded voltage.
Using the answer downstream
Record open-circuit voltage, loaded voltage, and load current with internal resistance. Include the operating state and data source.
Pass internal resistance with all available digits during downstream arithmetic. Round at the final reporting step.
What the formula leaves out
The source must be measured at a stable temperature and state of charge.
The equation does not include instrument loading, transient behavior, and component tolerance. Do not hide omitted behavior inside an unexplained correction factor.
Record conductor temperature and whether values are measured, nominal, or calculated. Solar DC Cable Loss Calculator covers the related DC cable loss step.
Common questions before using the result
How should I prepare the Source Internal Resistance inputs?
Use open-circuit voltage, loaded voltage, and load current from one operating condition. Keep source conditions with each voltage, current, resistance, or charge value.
What assumptions affect Internal resistance?
The source must be measured at a stable temperature and state of charge. A broader review should include reference-node errors, heating, and nonideal source behavior.