Lets get some real information before you run back to the white box auto parts retailer with another "failed" unit.

Has the charging system been tested on your vehicle?
What is the voltage at idle, 1,500 and 2500 RPM? What was the amp output at 1500 rpm?

Has the starting system been tested in your vehicle?
Starter amp draw?
Battery Load tested (volts and amps?)
Battery voltage at rest (key off)?
Battery voltage at idle?
Battery voltage at 1500 rpm?

These simple tests will tell anyone with auto diagnostic skills, what is going on. Everything else will be Scientific Wild Ass Guess > SWAG, based on personal experiences and other general guessing.

Notes:
Cheaply manufactured Alternators have approx 32% failure rate. A cheap Alternator with a cheap volt regulator installed makes that rate low. In a price driven world of remans, the installer is the first and last step in quality control.
Many posters have experienced what you are going through. It was their solution to replace the offending alternators with new Bosch units that resulted in solving their issues. IMHO the volt regs are the weak point on most of the white box alternators being sold at the auto part chains.

Stick with OE or OE Aftermarket as the original one lasted 100k.

__________________

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