Question

How do you calculate the internal resistance of a battery?

Answer 1

Hint: This question is just asking the formula for calculating the internal resistance of a battery, there is nothing to be calculated. Along with the formula, write the explanation about the formula which will help to understand the formula a little better. In the solution, an example can also be shown by assuming any value, but this is optional.
Formula used:
${{R}_{\operatorname{int}}}=\left( \dfrac{{{V}_{NL}}}{{{V}_{FL}}}-1 \right).{{R}_{L}}$

Complete answer:
When the power source delivers current, the measured voltage output is lower than the no-load voltage; the difference is the voltage drop (the product of current and resistance) caused by the internal resistance. The concept of internal resistance applies to all kinds of electrical sources and is useful for analyzing many types of electrical circuits.
A battery may be modeled as a voltage source in series with a resistance. These types of models are known as equivalent circuit models. In practice, the internal resistance of a battery is dependent on its size, state of charge, chemical properties, age, temperature, and the discharge current.
The internal resistance of a battery can be calculated from its open circuit voltage ${{V}_{NL}}$, load voltage ${{V}_{FL}}$ and the load resistance ${{R}_{L}}$, the formula for which can be written as-
${{R}_{\operatorname{int}}}=\left( \dfrac{{{V}_{NL}}}{{{V}_{FL}}}-1 \right).{{R}_{L}}$
In use, the voltage across the terminals of a disposable battery driving a load decreases until it drops too low to be useful- this is largely due to an increase in internal resistance rather than a drop in the voltage of the equivalent source.
In rechargeable lithium polymer batteries, the internal resistance is largely independent of the state of charge but increases as the battery ages; thus, it is a good indicator of expected life.

Note:
In this type of question, try to explain the formula as much as you can so that the solution is descriptive and can be understood easily. The working of the formula can be shown in the solution by assuming any value for substitution, but it is on the student on how they want to proceed, as mentioning an example is not compulsory.