An alternator will never generate more current than is needed by the demands of the electrical system
This is incorrect. If you full field an alternator, like the OP apparently did, it will put out maximum output regardless of load.
What controls the output of the alternator is the regulator and in the case of this setup there is no regulator and the alternator is hard wired to provide maximum output.
I once had a regulator fail on my and full field the alternator. Blew the over pressure caps off my Optima.
