i'm also having problems with this. i have an american autowire kit that is set up for a one wire alt, i have been persuaded to go with a stock alt with voltage regulator so i wire one in and without the battery hooked to the alt i'm showing 36v coming from the alt stud.
NEVER run a car with the battery unhooked. The voltage can run upwards of 100 volts and/or send out spikes that will burn out every electrical component in the car.