89 voyager SE diagnostic help

[connexion]

Hi,
I have a 4 cyl 1989 Plymouth Voyager SE and it is having problems. On a 40Km drive I noticed that the alternator charge display showed that the battery was losing its charge and I started to lose my wipers and lights and finally it quit on me. I assumed that it was the alternator as the van would not start after it died. As part of that assumption I figured that I had drained the battery via my lights and wipers.
However, after about 15 minutes of sitting it started up again showing a normal charge on the battery. Then, after another 8 minutes of driving the same thing occurred.
I had a friend come and give me a jump and I made it home.
This morning I went to put the battery on a charger to see what kind of loss I had, but, surprise to me, it was reading just shy of 12v and the van started fine after sitting the night.
The belt seems to be in OK shape and the alternator flywheel is spinning freely. I did a quick inspection of the wiring coming off the alternator and all seems to be in place and securely connected.

Could this be indicative of something slipping while the vehicle is under load (at idle all the flywheels seem to be functioning properly)? Would an alternator function intermittently? I always thought they either worked or didn't work.
Thanks is advance,
Dave

[RIP]

Should see around 14vdc across the battery posts with the engine idling. This indicates the alternator is supplying sufficient power to recharge the battery. That voltage may decrease slightly after startup or with many accessories on but should not go below 12.5 vdc.

Make sure the battery and engine/chassy ground connections are clean and tight. When it starts on a jump it indicates a weak battery. Have it charged and load tested. If the battery is over 4 years old consider buying a new one.

[connexion]

The battery came out at 12.5 and dropped to 12.28 with the vehicle running at idle. I assume this is the alternator? I have pulled the alternator out, and am wondering is there is a basic test I can run on it to see if it is the culprit. What I don't understand is how the battery could be running the car and still have a decent charge.
Cheers,
Dave

[RIP]

Autozone or similar stores will test the alternator for free.