What I am learning is that this test is NOT necessarily adequate in modern vehicles. At least in this 2012 Honda Civic, it seems that the power regulation system is pretty sophisticated. The voltage the alternator system puts out depends on the charge state of the battery. So it you have the system connected to a fully charged battery, you won't see any voltage increase at all. So when the battery gets drained, the right thing to do is to test the alternator THEN, as opposed to after you've installed a new, and fully charged battery, which is what I did. The belt tension can't have anything to do with it, unless the charging system isn't working. It is. The shop connected a power system analyzer to the battery, wherein you get load curves, battery status, and charging efficiency.