Battery volts drop

[Ron61]

That is not normal. when my fans and lights are on my voltage drops to about 12/13 volts. Check your terminals, have the battery load tested, and also check that your alternator is putting out a full charge. Running very long at less than 11 volts will cause the battery to discharge and leave you stranded.

Ron

[emm427]

Happy New Year to all as well ! ! First I would verify all your connections ( powers and grounds ) are clean and tight at the battery, starter and alternator . Then if you have a digital meter, start the car and verify voltage reads what the gauge is reading at the battery and the alternator, both with the lights off and then turn everything on. You might have a simple gauge gone bad, rare but is possible. A hour or so trouble shooting can save a lot of time running around to have parts tested that are still good. Good Luck !

[DanEC]

Assuming this isn't something that just started and it primarily drops at idle or very slow engine speed - then also look at your pulleys and make sure you don't have an underdrive pulley on your alternator. They are pretty common amongst the Mustang racers so one could have ended up on your engine.