# Internal resistance of a power source

Online calculator and formulas for calculating the internal resistance of a voltage source

## Calculate internal resistance

The internal resistance of a voltage source can be calculated by comparing two different load conditions. To do this, measure the open circuit voltage without load. Then the voltage source is loaded with a resistor and the loaded voltage is measured. With the determined data you can calculate the internal resistance of the voltage source on this page.

## Online calculator

Note the set unit of measurement.

Open circuit voltage

Internal resistance

Current through RL

Decimal places

## Formula for calculating the internal resistance

The internal resistance can be calculated using two load conditions.

The current with a loaded power source can be calculated using the following formula:

$$\displaystyle I=\frac{U_2}{R_L}$$

The internal resistance can then be calculated using the current and the voltage difference.

$$\displaystyle R_i=\frac{U_q-U_2}{I}$$

### Legend

 $$\displaystyle U_q$$ Source voltage $$\displaystyle U_2$$ Terminal voltage $$\displaystyle R_{i}$$ Internal resistance $$\displaystyle R_{L}$$ Load resistance $$\displaystyle I$$ Current with load resistance $$\displaystyle I_k$$ Short circuit current

### More formulas

Terminal voltage
$$\displaystyle U=U_q-(R_i·I_L)$$

Open circuit
$$\displaystyle U=U_q$$

Short circuit
$$\displaystyle I_k=\frac{ U_q}{R_i}$$