Have you read in the instruction book where it says that a discharge with the lights on is normal?, perhaps you need a good battery, i have gone many years with the same battery and never charged the battery other than by driving ---with 50% night driving, and I have quartz halogen bulbs ---I even turn on the light during the day on long trips to prevent overcharging ---modifications instead of proper restoration will not increase reliability ---has the generator been tested , do you have something drawing the battery down while the car sits??, does the battery hold a full charge?
At this time I am using a 12v battery cut in 1/2, it was handy when my 16 year old Optima died, I have to use the lights during day because I don't bother changing the 3rd brush, i have it set at about 14amps, it makes for a +- 1-2 amp with headlights on for the 35 watt halogen bulbs.
If you put the same sized pulley on a generator that an alternator needs to charge at idle the generator would charge at idle also
At -2 amps a good fully charged battery should drive the car all night, and at + 10 amps driven all day charge the battery to drive all night again.
Has the accuracy of the dash ammeter been checked??
If you have problems with the car or battery the alternator won't be the miracle you are hoping for.
Alternator conversions are good for the economy --more belt sales, more modified waterpump sales, more wire and cutoff switch sales ---
With Tom's regulator inside a properly working generator you wouldn't have to worry about adjusting the 3rd brush.