charging system question on 1999 Lincoln Continental

I need some electrical charging system advice> My 1999 Lincoln seem to have a faulty or limited charging system. I just changed batteries and traded for a higher quality that the one I replaced. It seems that the battery is not receiving a full charge and the battery -after only three weeks- shows low charge or need service when checked. The old battery had the same problem and left me several times stranded with no starting power. the day I changed batteries it started almost as soon as the key was turned, now it takes a couple of turns and when I took it to Jiffy Lube for service the battery was low.

there is no charge fault light that comes on the instrument panel, however I do understand that something is not quite right. Can use some guidance in troubleshooting and avoid doing a dozen expensive things by trial and error.
 
Response:
1) Ring out the voltage at the battery when the engine is idling. Should be between 14.2-14.4 volts. If low, check the belt(s) or alternator.


2) Also, if a serpentine belt, check the idler to see if the shaft is not bent, and the pulley runs freely.


3) you can take the alternator in to auto zone and get it tested free. Its usually the alternator when a car doesn't charge enough.
 
4) Yes 9 out of ten times is the alternator that has a bad diode and all of your energy leaks out, you can test by placing a screw driver on the back of the alternator while the car is not running and no key on the ignition if the screw diver is attracted to the alternator due to magnetism then you know to replace the alternator.