Theres fixed output alternators available, for example 28V fixed. They dont need a battery or anything.
Which alternator did you use exactly?
It may have a S or Ignition connection, each could be used to sense battery voltage. If they are not connected, then the alternator will raise its output voltage in an attempt to raise the voltage on the sense connection, to compensate for wire losses.
But as pwfarnell said, Orions are designed to be used as battery-to-battery, so even with a fixed voltage alternator theres no guarantee that it would work