Set the voltmeter to DC volts
Turn the engine 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
Keep the probes on the battery terminals
Read the voltage again with the engine running
Turn on headlights, blower, and other electrical loads
Read the voltage under load
Check for a reading around 13.5 to 14.8 volts with the engine running
Check for a lower reading if the alternator is not charging
Check for a higher reading if the voltage regulator may be faulty
Rev the engine slightly and watch for stable voltage
Inspect the alternator belt if the voltage is low
Check battery terminals and wiring for corrosion or looseness
Replace or repair the alternator if charging voltage is out of range