The job of your vehicle's alternator is to keep a constant flow of electricity to the battery. This charges the battery while driving and operates vehicle functions. When your battery dies, you should test the output of the alternator to determine what is causing the charging problem. A voltmeter will test the voltage traveling through an electrical circuit in both direct (DC) and alternating (AC) currents.
Set the voltmeter's electrical switch to DC.
With the engine turned off, open the hood of your vehicle.
Hold the end of the red lead coming from a voltmeter to the positive post of the vehicle's battery. Hold the black lead coming from the voltmeter to the negative post of the battery. The voltage should be displayed on the voltmeter's screen. Remove the leads from the battery posts.
Start the vehicle. Again, hold the leads of the voltmeter against the battery posts. Read the amount of voltage displayed. A properly functioning alternator should show a battery voltage ranging from 13.5 to 14.2 volts.
Rev the vehicle's engine to 1,500 rpm. Take a third reading across the battery's terminals. The voltage should increase if the alternator is functioning properly.
Replace the alternator if the readings remain at or are below 12 volts.
