Question

Your flashlight is getting dimmer and dimmer, so you assume the batteries are nearly dead. They are “D” cells producing 1.5 volts. You take the batteries out and test them with a voltmeter. The meter reveals the batteries have nearly 1.5 volts of potential. Are the batteries dead or is something else going on? Discuss how you can tell whether the batteries are dead, and how you would test the flashlight to see whether there is some internal problem. Is it an either-or proposition? Explain your answer in detail.

Answer 1

If we disconnect the battery and measure the voltage across the battery terminals, we will get 1.5V

As a battery gets older its internal resistance get larger. The total resistance of the circuit increases and reduces the current flowing through the circuit. As the current gets smaller the flashlight gets dimmer.

In a dead battery the internal resistance has become so large that the battery can no longer produce a measurable current. A good voltmeter does not draw much current, so it can still measure approximately the electromotive force of the battery, even if the battery has too much internal resistance to produce a measurable current.