I have a brand new alternator, but it won't hold the charge. Why?

I have a brand new alternator, but it won't hold the charge. Why?
Added (1). The car is a Honda Civic EX 2001 1.7 4 Cylinder. Brand new alternator and battery is still working I recently yesterday got it fully charged at the shop and the alternator isn't holding the charge. Why?
Added (2). The alternator is BRAND NEW I just went to the shop and got it bench tested. The alternator works.

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