Set the multimeter to DC volts
Turn off the engine
Connect the red probe to the battery positive terminal
Connect the black probe to the battery negative terminal
Check the battery voltage with the engine off
Start the engine
Check the battery voltage again with the engine running
Look for a reading around 13.5 to 14.8 volts
Turn on headlights, blower, and other electrical loads
Check the voltage again
Make sure the voltage stays above about 13 volts
Set the multimeter to AC volts
Check the battery with the engine running
Look for low AC ripple voltage
Test the alternator output terminal if accessible
Compare the reading to the battery voltage
Inspect the alternator belt for looseness or damage
Check for blown fuses or loose connections
Replace the alternator if voltage is too low or too high