I think the alternator testing machine was wrong. 218K miles on the original alternator is more than I've ever seen on this forum. Mine went at 126K. With the engine running, you should be seeing something like 13.8 volts at the battery posts.

2002 X5 3.0 312,300 miles
2014 428i 22,800 miles

2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles

1970 Firebird - Under restoration