SHELBY GT 350 Member
Joined: Sep 2010
Location: see user name
Sounds more like alternator. But, the only way to find out for sure is have them both tested. Honestly both could be bad - if the alternator is weak the battery has been taking up the slack. FWIW, Optima batteries don't live up to their hype or price tag anyway. Have it tested, replace if necessary (my hunch says 'yes') and go from there.
2001 GT Convertible
All the usual bolt-on's /200 I6, C4
And a few Explorers too... 2008 4.0 XLT 4x2, 1996 XLT 5.0 4x2, 2003 Sport Trac XLT 4.0 4x4