Have you checked the voltage while charging? You would want to check both accross the battery posts and accross the alternator positive and negative connections. Do the battery check and then do the alternator check. Voltage should be above 13.5 volts when the car is running. Sometimes corrosion builds up on the contacts and while the alternator light may not come on the alternator won't charge the battery due to poor connectivity.

I'm not as familiar with the 850 electrical system, on some of the older models cleaning the primary ground connection can make a difference.

Another thing to check is the battery cables, they may be badly corroded and cause this type of problem as well.

I hope this helps.