Charging test...
Here's what you need to do.
1. Engine off - battery standing voltage.
With the engine off and fully charged battery, hook up the meter directly to the battery terminals(not clamp, bolt, nut, etc...). Read the meter as volts DC. The voltage will depend upon the charge level of the battery, battery age and ambient temperature. 12.6 volts is considered 'normal'. Dry cell batteries (Optima and others) often have a lower standing voltage than typical wet cell batteries. Cold batteries will obviously have lower voltage readings, but a battery heat-soaked from engine temperatures will also read lower than normal.
2. Engine running - low idle - no load charging voltage.
Engine running at idle, all lights, accessories and AC off. Read the meter after voltage stabilizes from starting engine. Typical should be above the battery standing voltage by at least 0.5 volts (example 13.1 volts vs. 12.6 volts).
3. Engine running - hi idle - no load charging voltage.
Engine running at 1500 RPM, all lights, accessories and AC off. Read the meter. Should be higher than low idle no-load charging voltage but no more than 2 volts above battery standing voltage (example max 14.6 volts).
4. Engine running - full load charging voltage.
Engine running at 2000 RPM, lights on high beams, AC on MAX(blower on high). Read the meter. Should be between the low idle no-load charging voltage and the high idle no-load charging voltage (for example between 13.1 volts and 14.6 volts).
These readings give a good indication of the charging system while on the car. As it has been repeated many times, a bench test is no substitute for an on-vehicle test. Also keep in mind that a 'cold' alternator performs better than a 'hot' alternator. Alternator output decreases with an increasing alternator temperature. So testing 'hot' will yield less than desirable results.
Any alternator test assumes a fully charged battery and no wiring deficiencies. The factory battery terminals are notoriously cheap and subject to problems. Also check the battery cables for signs of corrosion. Any visible corrosion is a dead ringer for a bad cable as it is likely corroded internally far worse than the external indication. Also check the charging cable from the output terminal of the alternator. It is highly recommended to run an additional 4 gauge or bigger fused wire from the output terminal of the alternator to the positive stud on the power distribution box (under hood fuse box) or the positive battery terminal.
Often overlooked is the 20A alternator fuse in the engine compartment fuse box and the regulator harness connector on the alternator. I have seen many flaky connections with the regulator harness connectors on Ford vehicles. A 'wiggle test' will generally confirm this. Replacement harnesses are readily available to splice in place of the OEM harness.
Verify any alternator purchased (even from a dealer) is a "new" unit and not a reman. Inventories change all the time and remanufactured units are often sold as "new".