[TheCabinetmaker]

You need to excite the field coil in the alternator with the car battery in order to get the alternator to work. No current in the field coil and she wont generate any juice.
The small field current is brought to the alternator's rotor by brushes, but unlike generator brushes, carry little current and hence rarely fail. Generators have brushes to carry the charging current, which can cause brushes in generators to wear out. Brushes are a regular maintenance item on generators not alternators. Alternators are considered maintenance free for the most part because of this

the 914 battery is made of six individual cells, each providing about 2 volts of juice, all six are arranged in series to get the 12 volts in a car battery (three cells are in a six volt car battery) If one of the cells shorts out, then you effectually have a ten volt battery instead of a 12. No matter how hard the alternator tries, it cant charge a 10 volt battery(five cells working) to 12 volts. That would be like expecting a good 6 volt battery (three cells working) to be able to be used in the 914 and run at 12 volts, it ain't going to happen.

sometimes cells die as an open rather than a short, in that case the entire battery will be zero volts, as the open cell effectively breaks the series connection of the cells.

PS after an over night charge on a known good home charger the offending battery never got up to 12 volts. Thus it was found to have a dead shorted cell. The new battery is charging fine in the car, the car's charging system is working.