Set the multimeter to DC volts
Turn the car off
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
Measure the battery voltage again with the engine running
Check that the reading is about 13.8 to 14.7 volts
Turn on headlights, blower, and rear defroster
Measure the voltage again under load
Check that the voltage stays above about 13.5 volts
Rev the engine to about 2,000 RPM
Watch for stable voltage without large drops or spikes
If voltage stays near 12 volts with the engine running, test the alternator output
Set the multimeter to AC volts
Measure across the battery terminals with the engine running
Check that AC ripple is low, usually under 0.5 volts AC
Inspect the alternator belt for looseness or damage
Check battery terminals and alternator connections for corrosion or looseness
If voltage is too low or too high, replace or repair the alternator or charging system