Some alternators will self excite on what is termed residual magnetism,but in just about every case when an alternator is "open circuited" i.e. the battery disconnected,the regulator has nothing to referance the output voltage to and therefore the output can for instance on a 12 volt unit end up around 30-40 volts,a 12 volt,80 amp alternator can be used as a welder but with a low duty cycle,that being infrequent use with extended cooling times but once again this is a very crude way of doing things.As one of the other members has pointed out they do take a bit to drive and this will increase with the load placed on the alternator,happy experimenting
