Suddenly no juice

The fuse rating has to be large enough to last through a surge of up to the max amerage rating of the alternator and not blow. With a slow blow, the fuse can take amperage well OVER the fuse's rating and not blow for some period of time. Here is the link to the Maxi-fuse dataq sheet:

[ame]http://www.littelfuse.com/~/media/automotive/datasheets/fuses/passenger-car-and-commercial-vehicle/blade-fuses/littelfuse_maxi_32v_datasheet.pdf[/ame]

It shows that at 135% of rated amperage, it takes from 1 minute to 30 minutes to open. So with a heavily discharged battery, how long will the alternator try to shove 60A to the battery??? It would have to be longer than a 40A fuse's blow time rating at 150% of rated current.

A fusible link is not anywhere rated at 60A; it depends on the same thing to open: heating that will blow it but if the overload % is low, it will take a long time.

To the OP, I would advise replacing the fuse with a fusible link not too long from now. The weak link in your temporary setup the holder; it will tarnish and then heat up and then lose contact and springiness to grip the fuse blades. And your experiment is interesting, but only valid if you try it with the fuse loaded to maximum, which requires that you have a heavily discharged battery each time you try it. Next heavist load through the link is likely with the car not running and headlights, heater blower motor, etc, turned on.

Glad you found the issue !