Demos: 5B-02 Voltage at the Terminals of a Battery

A high resistance projectable voltmeter is connected across a battery, which is in series with a 100W resistor and switch. When the switch is closed, the terminal voltage decreases.

Directions: The battery used is an old dry cell that is very weak. With the switch open, the voltage across the battery terminals is close to the rated voltage, but when the switch is closed, the relatively large internal resistance of the battery causes the voltage at the terminals to drop considerably.

Suggestions for Presentation: Discuss the problem of trying to determine whether a battery is good by testing it with a voltmeter. Place the voltmeter across the battery terminals and show that it appears to be good, i.e. it shows the rated voltage of the battery. However, when a load is introduced into the circuit, the voltage drops drastically. Let this lead into a discussion of internal resistance.

Applications: Battery testers are reliable only if the battery is placed under a load when testing.

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