Start the car, while running disconnect the battery positive terminal.
1. If the car dies, the alternator is NOT charging.
2. If the car RPMs goes up, the battery or car may be pulling it down.
Read the battery with the terminal disconnected. If it is 12.8 to 13.2 volts, you battery is probably fine.
Read the voltage at the wire you connect to the battery. it should be around 14.2 to 15.1 volts. If it is over this, it may be over charging the battery. If it is less, I would have the alternator checked. It can put out just the minimum and not charge the battery. This is caused by either the feedback wire ( self excitation ) not connected properly to the battery charging system. Bad regulator ( not grounded well in the case ) or winding failure ( I think, it's hard to remember this stuff when you haven't used in in several years
).
Single wire or self excitation works as long as all of the criteria are met. Personally I use the three wire setup as designed by Ford. It is a no brainer for anyone if they just settle down when working on it.
Just my $0.02