First off try to Polarize correctly by momentraily flash jumping from BAT on the VR to ARM/GEN NOT to FLD...... See my URL below about dead grounding the FLD test and Motor test, you dont have that quite correct....
Next was it originally Pos or Neg ground?? It can still charge at EITHER polarity provided its correctly Polarized however SUBJECT TO if the ammeter isnt wired right it will register just bass ackwards. (see my Troubleshooting Procedure URL below for more info)
Youre correct, if coupled to a good working charging system at fast RPM the batetry voltage should rise from 12.6 volts up to at least 13 to 14 and over depending on battery and genny and RPM