THere are two possible realities.
THe first is that the battery could have a bad cell. This is possible even with a new battery. The second is that the alternator could have a bad primary diode allowing voltage to flow back into the stator windings.One wire systems are somewhat less problemfree when compared to the 3 wire Delcos, or Hitachi. Three system checks are needed. The first is to determine the actual charging voltage.
Running at High Idle (wide open) check the battery voltage across the battery terminals (use a analog meter if possible, they are better when looking at old unshielded machines)
The charge voltage into a fully charged battery should be between 14.2, and 14.7 volts. Anything over 15 volts is an issue with the internal regulator in the alternator.
If OK, check the discharge of current into the alternator (bad diode as above. With it not running, and all wires connectd, remove the wire connecting the alternator to the wiring harness at the alternator, be careful, this wire is connected to the battery and cannot be touched to grounded things. (doing this at that location reduces the chance of making a spark where hydrogen could be found near the battery. If this wire makes a spark when touched to the post you took it off of, the diode bridge in the alternator is bad. If it does not spark, use a test light (not the meter) to see if current is flowing between the wire and the terminal you removed it from. The light should connect to the wire, and be touched to the terminal. If no light, not even a glow, the diodes are probably OK.
Next check the battery (a hydormeter (specific gravity reading of each cell is great, they should be equal in specific gravity from cell to cell, and represent the state of charge of the battery from a chart) OR after running check the temperature of each cell with a glass thermometer to determine differences. Be careful with acid. The temps should be just a bit higher (10 to 15 degrees in the center cells, and a bit less on the two end cells due to being exposed to more outside cooling. If celld are 30 to 50 degrees or more different, the battery is bad.
Good luck, and keep us informed as to what you find. JimN
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