5.4 Internal Resistance

This is the fourth week of Module 5 in the Navy Basic Electricity and Electronics series. This module is on the relationships between current, voltage, and resistance. The first three sections were:

5.1 Voltage, Resistance, and Current
5.2 Ohm's Law Formula
5.3 Power

1. This week is on the internal resistance of a source such as a battery. Let's say that you were to put a voltmeter across a battery in an open circuit and it were to read 12 volts, the "no-load" voltage. Then lets say you close the circuit and the voltmeter drops to 11 volts. This suggests that your battery has an internal resistance that is zapping some of the force.

2. To measure the internal resistance (R_i) of a source:

• Measure the no-load voltage.
• Energize the circuit and then measure the voltage across the source again.
• Subtract the difference (internal resistance decreases the voltage in the circuit).
• Measure the current in the circuit.
• Use R=E/I to determine the internal resistance of the source.

3. Hopefully, the internal resistance of your source is small enough to be negligible.