Open Circuit Voltage: What is it? (And How To Find And Test It)

What is Open Circuit Voltage?

Open-circuit voltage is the potential between two terminals with no load connected, so no current leaves those terminals. Call that the voltage of an open circuit at that pair of nodes. In linear network analysis it is the Thevenin Voltage. Symbols: OCV or V_OC.

With the load off those terminals, no electric current flows in the load. Current can still flow in other loops of the source network.

With a load on, terminal voltage is less than the source EMF by internal drop. With the load removed, I = 0 in that branch so internal IR is zero. For an ideal voltage source, Voc equals the source voltage . A real battery’s rest voltage is its EMF, not a leftover “small drop.”

Voc is a standard rating on solar cells and batteries. It depends on temperature, state of charge (after rest) and, for a cell, illumination.

How to Find Open Circuit Voltage?

Find Voc as the voltage between the two open terminals, by measurement or by circuit analysis (Thevenin).

If the only load is removed from an ideal source, Voc equals the source voltage. There is no IR drop inside the source at I = 0. The stored “small drop across the battery” sentence does not apply to a true open circuit.

If other branches stay connected, Voc at the open pair is the Thevenin voltage of what remains, not the raw source voltage. Example:

In the stored figure, resistors A, B, C and a load sit on DC source V. Open the load at P–Q.

Voc is V_PQ. If A and B are the series path across V, loop-1 current follows Ohm’s law as in the stored line.

   

That current is the series current through A and B (read from the stored figure).

   

The open branch through C carries no current, so C drops 0 V and does not affect Voc at P–Q in this layout.

If P–Q is across B, Voc equals the drop on B:

   

That V_b is Voc (Thevenin voltage) for this example. 

Open Circuit Voltage Test

A Voc test measures rest voltage between the terminals of a battery or PV string with no load.

Batteries convert chemical energy to electrical energy. Primary cells are not meant to be recharged; secondary cells are.

Both types can be checked at rest. For many rechargeable chemistries, rested Voc is a rough SOC clue, not a full capacity test.

Use the maker’s datasheet. The printed “12 V” (or similar) on a case is a nominal voltage, not the exact rest Voc (a charged 12 V lead-acid cell string is often near 12.6 V to 12.8 V).

Measure with no load. Isolate the battery if the circuit would still draw current. Leave the cell at rest so surface charge dies down.

Set a digital multimeter to DC volts and read across the terminals. A low reading often means the pack is discharged. It does not by itself prove the battery is damaged. A loaded capacity test is the next check.

On rechargeables, rested Voc hints charged vs empty. Health still needs a capacity (amp-hour) test under load.

Why is Voltage Not Zero on an Open Circuit?

Voltage is potential difference. Two nodes can sit at different potentials even with no conducting path between them.

In the usual Voc setup the open terminals still see a source. The source’s two poles are at different potentials, so a voltmeter between the open terminals reads that difference.

So Voc is not zero just because current is zero. Current zero and voltage zero are different statements. 

Open Circuit Voltage of a Solar Cell

On a PV I–V curve, Voc is the voltage at I = 0. That intercept is the highest voltage on the curve for that irradiance and temperature, not the maximum-power voltage Vmp.

Absorbed photons generate photocurrent. Voc is the forward bias where the diode current cancels that photocurrent, so net terminal current is zero.

The stored I–V figure marks Voc on the voltage axis.

A common single-diode expression is:

   

Where,

I₀ = Dark saturation current
I_L = Light generated current
N = Ideality factor
T = Temperature
k = Boltzmann constant
q = Electronic charge

The NkT/q factor rises with T, so the formula can look as if V_oc rises with temperature. I0 rises much faster, so silicon V_oc falls with temperature (about −2 mV/°C per cell). The net measured trend is a decrease. 

Open Circuit Voltage Example Questions

Finding Voc is the same procedure as finding Thevenin voltage. Two teaching examples follow. Arithmetic in the stored working is retained.

Example-1

Load R_L is on a DC source. Remove the load. The next figure is that open network.

With the load open, current in the 10 Ω branch is zero. Voc equals the drop on the 3 Ω resistor in the stored figure.

KVL around loop-1 (stored working):

   

   

   

Drop on 3 Ω:

   

   

   

Example-2

Disconnect R_L and find voltage at those terminals. Next figure:

In that figure Voc is the drop on the 6 Ω resistor, so find the current through 6 Ω.

KVL on the outer loop (stored):

(1)  

The 3 A source in loop currents (stored):

(2)  

Solving those two stored equations gives I₁ and I₂. Current in 6 Ω is I₂. Check: I2 − I1 = 3 and 24 = 3 I1 + 6 I2 ⇒ I2 = 3.667 A, Voc = 22 V.