One, you're welcome.

Two, I see this happen often - especially when it's electrical or fuel. Guess, guess, guess. Comments like "sounds like a bad [ fill in name of expensive part here ]" from people that have no clue how [ expensive part ] works or how to check it. Electrical problems, particularly when not intermittent, can usually be diagnosed PROPERLY in minutes. Often before replacing anything. >> Disclaimer: electronic failures, not always so easy.

Three, once battery charging voltage has been found to be low (and idle isn't necessarily a fair test), checking AT the alternator should always be step two.

Now you understand why I was very specific about where to hold the meter probes through the testing. It eliminates the very-often-wrong assumption that metal grounds, fat wires and and big clamps all mean good connections that don't have to be checked.

Four, the positive output wire goes down to the starter on your car. Disconnect the battery (probably the umpteenth time) and clean both sides of the two terminals on the starter stud. Your voltage drop is probably oxide on the facings of the smaller alternator wire terminal that is sandwiched between the battery cable and the base of the stud. However, if you can maintain 13.75V or higher at the battery with lights on and engine at 1500 or higher... shut the hood and leave it alone.