Yep it is the charging system. I do not know any way to determine Alt brand other the visual confirmation. Maybe others can offer suggestions.

Smack it with a rubber mallet and see if the brushes connect and it starts charging once again. Brushes wear old and build up carbon tracking which in turns into a charge on charge off condition that will fry most Regulators.

Options:
Replace with new
Rebuilt what you have
Replace with a proven re-Manufactured unit (Before you leave the store, make them test that it actually works, Ask for the amp and voltage readings , not just a green light that says "OK"))

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green