Multi-Stage Regulators | Balmar
I can't seem to copy from the page above so I pasted the link. Check it out.
The ARS-5 has a maximum field current
of 9 amps. Most of the other balmar
regs will put out 15. I'll bet you an IPA that your big ole Powerline needs 20 amps of field to reach full power. It's beefy as all get out, but not particularly efficient. With 9 amps of field, the best output you can get is 70. So the pot is giving you more field current
in order to make more output amperage.
You could get a reg with higher field capacity, a more efficient alt, or just keep doing what you're doing. Nothing is broken, and your alternator
doesn't seem to overheat, so...