Formula used
- Loaded measurement: Voc = Vload + Iload·Rint, with Iload = Vload/Rload.
- Norton/Thevenin: Voc = Isc·Rth.
- Photovoltaic diode model: Voc = (n·k·T/q)·ln(Iph/I0 + 1). Constants: k = 1.380649×10−23 J/K, q = 1.602176634×10−19 C.
How to use this calculator
- Select the method that matches your measurements.
- Enter values and choose the correct units for each field.
- Click Calculate to display results above the form.
- Use Download CSV or Download PDF after a successful calculation.
- For accuracy, use consistent conditions (temperature, load, and measurement setup).
Example data table
| Scenario | Inputs | Computed Voc | Notes |
|---|---|---|---|
| Battery under load | Vload = 11.4 V, Rload = 4.7 Ω, Rint = 0.25 Ω | ≈ 12.006 V | Uses Vload + (Vload/Rload)·Rint |
| Thevenin/Norton | Isc = 2.4 A, Rth = 5.0 Ω | 12.0 V | Linear source approximation |
| Solar cell (diode model) | n = 1.2, T = 25 °C, Iph = 3.5 A, I0 = 1×10−10 A | ≈ 0.792 V | Uses (n·k·T/q)·ln(Iph/I0 + 1) |
Open Circuit Voltage: Practical Guide
1) Meaning of open circuit voltage
Open circuit voltage (Voc) is the terminal voltage of a source when no external current is drawn. Because current is essentially zero, conductor drops are minimal and the reading reflects the source’s internal electrochemical or electromagnetic potential under that condition.
2) Why Voc differs from operating voltage
The moment a load is connected, current flows and internal resistance and contact resistance cause a drop. A 12 V battery can show 12.6 V open circuit but sag to 11.4 V at several amperes; the difference is valuable for judging health and available power.
3) Fast field measurement tips
Use a high‑impedance meter, clean probes, and allow sources to settle. For batteries, rest time after charging or heavy discharge reduces surface‑charge effects. For capacitive supplies, wait for transient decay so the reading represents steady open‑circuit behavior.
4) Thevenin and Norton interpretation
Many sources can be approximated by a Thevenin model: an ideal voltage source in series with Rth. In this view, Voc is the Thevenin voltage. If you know short‑circuit current Isc, then Voc = Isc·Rth.
5) Estimating Voc from a loaded test
When you can safely apply a known load, measure Vload and compute Iload = Vload/Rload. If internal resistance Rint is known or estimated, Voc ≈ Vload + Iload·Rint. This is common for batteries and small power supplies.
6) Solar cell Voc behavior
For photovoltaics, Voc depends strongly on irradiance and temperature. A simplified diode model gives Voc = (n·k·T/q)·ln(Iph/I0 + 1). At 25 °C, n≈1.2, and Iph≫I0, the log term dominates, so Voc rises slowly with light but falls as temperature increases.
7) Temperature and material effects
Metallic sources and conductors show resistance increasing with temperature, which increases voltage sag under load (but not necessarily Voc). Semiconductor junction sources often have a negative temperature coefficient for open‑circuit voltage, so warmer cells typically show lower Voc.
8) Using Voc in design and troubleshooting
Compare Voc to rated values to detect depleted batteries, poor connections, or mismatched PV strings. Pair Voc with load testing to estimate internal resistance and predict regulation margin. In power electronics, ensure worst‑case Voc stays within component voltage ratings.
FAQs
1) Is Voc the same as the rated voltage?
No. Rated voltage is usually under a specified load or operating point. Voc is measured at (near) zero current, so it can be higher than the operating voltage for many sources.
2) Can I measure open circuit voltage with any meter?
Use a high‑impedance digital meter whenever possible. Low‑impedance testers draw current and can under‑read the true open‑circuit value, especially on small or weak sources.
3) Why does Voc change after charging a battery?
Immediately after charging, surface charge can elevate the reading. After resting, the voltage settles closer to the true open‑circuit state. Temperature also shifts the measured value.
4) How does internal resistance affect this calculator?
In the loaded‑measurement mode, internal resistance converts load current into an estimated internal drop. Higher Rint increases the difference between Vload and estimated Voc.
5) Is short‑circuit current safe to measure?
Not always. Some sources can deliver very high current and overheat conductors or damage components. Only measure Isc when the source and test method are designed for it.
6) Why is PV Voc temperature‑dependent?
Junction physics changes with temperature; the saturation current increases as cells warm, which reduces Voc. Cooler panels typically show higher open‑circuit voltage under the same light.
7) What should I do if results look unrealistic?
Recheck units, confirm resistance values, and verify measurements. Ensure the load is known and stable. For PV mode, use realistic I0 values and correct temperature in °C.