Pull the Pos. battery cable off while running and see if it shuts off. If it stays running start turning things on one at a time until it shuts off. You found the draw.
Steve I'm surprised at you. This is a fake "test" and can cause damage to modern electronics.
It also is not really testing anything, because, if the engine happens to be running slowly enough that the "good" charging system is not putting out much, the engine may die and lead you to falsely believe that it is not charging
If the alternator is merely "down" from full output, like, maybe, 2 or 3 or 4 open diodes, the engine may keep running and lead you to the false conclusion that the charging system is OK
