A voltage source can be modeled as an ideal voltage source in series with a resistance, Rint as shown below:
The internal resistance of a voltage source (Rint), can be determined by placing a known resistance or R across the open terminals
of the circuit shown in the figure above.
VOC is the open circuit voltage (5V in this example) VR is the voltage across the same terminals when an external resistor, 1k, is connected (V= 2.5V in the example)
Rint = R(VOC - VR) / VR
For the example shown:
Rint = 1k(5V - 2.5V) / 2.5V = 1k