A battery load tester checks if a battery can handle the heavy current needed to start an engine. Standard multimeters only measure static voltage, which can be misleading. A bad battery can show 12.6 volts when sitting idle but drop to 5 volts the moment you turn the key. The load tester uses a built-in heating element to draw a heavy electrical load (usually 50 to 100 amps) for 10 to 15 seconds, mimicking the starter motor load.
To run the test, connect the heavy-duty clamps to the battery terminals. Press the load switch and watch the analog or digital voltmeter scale. If the voltage drops and stays below 9.6 volts during the load cycle, the battery has failed and can no longer hold a charge under stress. The heating coil inside will get hot and smoke slightly during the test, which is normal as the tool burns off dust.