83 320i charging system problem

I cannot figure out why my battery is not charging. I know the alt is connected correctly. I replaced battery cables, grounding strap, and the alt is a rebuild. The belt is new and tight, but not too tight. I had the alt tested and, according to the place I had it tested, it is throwing a charge. Yet when I throw a meter on, the battery shows no change in voltage, besides it dropping due to lack of alt charging. I am getting tired of seeing that little red battery light. I have worked on this car for months to get it in working order, and this is all I can't figure out. I would appreciate any info you can give me.

Thanks in advance,
Nick

83 320i
86 Monte Carlo SS