My brothers focus ST broke down again. After replacing the alternator yesterday i went for a drive after about 50 miles driving the alternator possibly broke again. The battery doesn't charge and the battery light on the dash is on.

The first time (a week ago) my brother stranded with this car because of an empty battery. The alternator did nothing and the battery light on the dash was off!!!

what could this be that the alternator doesn't charge and broke again within 50 miles?