Check the battery voltage at the battery terminals, engine off. Should see ~12V
Check again with engine idling, then again at about 3000 rpm, high beams on, then off.
Should see ~ 14.3V.
Place your meter's black lead at various places to compare. Try radiator support, then engine block, then fender. All voltage readings should be the same, whether you are using the - battery terminal for your black lead (ground) or the block, rad support, or fender. If they vary significantly, repair your grounds.
Most auto parts stores will test your battery and alternator IN THE VEHICLE for no charge.