That doesn't typically affect the Duratec engined Focis. It is possible though, but I don't think that is your problem. I think you have a bad voltage regulator. If you were getting those readings using the electronic odometer trick then you were getting readings that were .2v lower than what can be read at the battery. Anything over 14.8 is bad news, so I'd say anything over 14.6 at when read using the electronic odometer trick is bad news.
Have you ever replaced your alternator before? If you DIY, I recommend only Motorcraft new, or rebuilt, check RockAuto, but have your VIN handy. I'd then compare that part cost + shipping to the local dealer and our OEM dealer Village Ford in the Vendor section. Keep your core if you plan on keeping your car. Take your core to a local auto parts rebuilder and have it rebuilt. That is another recommendation I make a lot. Use local rebuilders whenever possible. Usually the part will last for the lifetime of the car whereas the "lifetime warranty" parts at big chain stores won't last 4 months. Most people can't be without their vehicle for a day in order for a local rebuilder to rebuild it.
Completely unrelated, but worth mentioning== have you ever replaced your spark plug boots? Those do go bad after a while. The typical life I've observed is 80-100k miles or 5 years. It is probably just as relative to age as mileage. Boots typically cost $3-8 each, and are very easy to replace.