I'm not sure what you mean by,
"20 amps on start, and the voltage at the circuit board is 17 volts which falls to 15 volts on running."
What do you mean by, "on start"?
Do you have a "single wire alternator", or do you have an alternator that is wired improperly?
The standard 10SI or 12SI has an internal regulator that needs to be told what the battery needs, so that it can adjust the alternator output accordingly.
