well,there is a place here in my area ,from what i know ,one of my family member ,had some problem like you have ,voltage,and he replaced it with a used one for 150$ with 6 or 12 month warranty (not sure 6-or-12)...

my alternator ,everything was fine and then one day i start hearing some big noise coming from the front, in my case a bearing was the problem,but i put new one for about 350$ with lifetime warranty at riebiest auto parts.

goodluck,friend

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2017 Bmw X5 F15