Thanks for the test numbers.
voltage at idle is 13.5v-13.3v
at 1500rpm its 13.9v-13.7v
at 2,500rpm if the light isn't on about the same as 1500 rpm
if it is on drops to low 12v.

Based on the fluctuation above idle I would return the unit, stating that there is a loss of charging state at hwy speeds and demand a replacement. Then I would make them test the unit before leaving the store. The test needs to show Voltage and total amp output at multiple RPMs. It can not be just volts as that will not show total output.

The numbers show that at idle your alternator is charging correctly.
ANY fluctuation off idle and into higher RPM shows a slip ring, windings loose, or brush contact issue. VERY common issue on rebuilts that have not been cleaned, ground flat, and paint over spray on the contact points. This problem with rebuilt units has been a re-manufacturing issue since the 60's as the cost of quality control and true manufacturing brings the cost to the end user close to new prices. That is why there are so many short cuts and defect units. Once you had the chance to bench test a few defects, you will notice that all some do is clean, paint, and slap a regulator on them after a quick cleaning of the stator along with a bushing or two. A REAL BAD PROCESS but one that delivers a cheap unit with controllable defect rate.

If it was my vehicle or a customer vehicle, I would install a new Valeo unit of the highest AMP output I could find, as it would be 100% new and bench tested. In your case as you are doing the labor each time, you get a simple choice...price or performance reliability.

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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