Turn off the engine and inspect the alternator belt for cracks, looseness, or wear
Check the battery terminals for corrosion or loose connections
Start the engine and observe the dashboard battery warning light
Use a multimeter to measure battery voltage with the engine off
Measure battery voltage again with the engine running
Compare the running voltage to the normal charging range
Turn on headlights, radio, and air conditioning, then recheck voltage
Listen for whining, grinding, or squealing noises from the alternator
Look for dimming or flickering lights while the engine is running
Test the alternator output with a professional battery and charging system tester
Have the alternator inspected if the voltage is too low or too high