Set the multimeter to DC volts
Turn the engine off
Measure battery voltage across the battery terminals
Start the engine
Measure voltage across the battery terminals again
Check for a reading around 13.8 to 14.8 volts with the engine running
Turn on headlights, blower, and other electrical loads
Measure voltage again with the engine running
Check that voltage stays above about 13 volts
Set the multimeter to AC volts
Measure across the battery terminals with the engine running
Check for low AC ripple voltage
Inspect the alternator belt for looseness or wear
Check alternator wiring and connections for corrosion or damage
If voltage is low or unstable, test the alternator further or replace it