Age old question...Bad Battery or Bad Alternator???

Had some time today to work on her and the battery was dead...reading 11.50 v . I had the battery tested and she would hold more than a 25% charge. Sooo I bought a new battery and as we all know that wasn't the issue. She fired right up and I was only getting around 12.15 under load / no load @ 2000 rpms. All contacts to alternator / voltage reg. are fine and were also cleand with some steel wool. I will update tomorrow with some additional test.