Set the multimeter to DC voltage
Turn off the engine
Place the red probe on the battery positive terminal
Place the black probe on the battery negative terminal
Read the battery voltage
Start the engine
Keep the probes on the battery terminals
Read the voltage again with the engine running
Check for about 13.8 to 14.7 volts
Turn on headlights and blower
Read the voltage with electrical load applied
Check that voltage stays above about 13.5 volts
Set the multimeter to AC voltage
Keep the engine running
Measure across the battery terminals
Check for low AC ripple voltage
Inspect the alternator belt for looseness or damage
Inspect battery terminals for corrosion or looseness
If voltage is too low, test the alternator wiring and fuse
If voltage is too high, check the voltage regulator
If the battery voltage does not rise with the engine running, replace or repair the alternator