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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
Best you could do is checking the voltage regulator on the back side of the alternator. Usually when those go bad, batteries either overcharge or undercharge constantly, in my case. Try replacing it.