The alternator makes and alternating electric current (AC), but the car uses non-alternating current (DC).
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the car's charging wiring, including wiring to the ammeter in the dash, is not large enough to handle the increased current.
But that's a subject for about a hundred other pre-existing threads.
I would have simplified this greatly, and added, that the car's alternator makes AC INTERNALLY in the alternator. An alternator normally does not output anything except DC. That is, simply and practically speaking.
The too large vs stock alternator is a danger to stock wiring, and is why you must examine, repair, or bypass the ammeter circuit. THAT IS certainly for later discussion