11.1 volts at the battery when running
 

My less than 1 year old battery seems to have 11.1 volts when running as reported on my Foxwell scanner Live Data test.
The battery light is on. The battery light just came on and went off. I ran the Live Data test earlier today and the results showed 13.4 volts and the battery light was off. Is this an indication of a dying alternator or slipping belt? On the freeway at 70mph, the battery light was still on. During one test session, I revved the motor and the voltage went from 11.1 up to 13.4 and the light went off. Any thoughts? The X5 did sit for a bit longer than usual during a two week warm spell when I drove the 335.

You need a new alternator.

Sounds like your problem is intermittent, assuming the measurements are all accurate. That is common when one of the brushes in the alternator (specifically, within the voltage regulator, which is within the alternator) wears down to the very limit. Sometimes the brush is still long enough to contact the slip ring, and sometimes it's not.

That would be the place to look. Requires removal of the alternator, pop off the back cover, pull off the VR/brush assembly and see how long things look.

A slipping belt is possible, but I think you'd hear / smell / see other symptoms if that were happening.

Ok this is real easy
Get yourself a digital volt meter one with min/max will make your life easy
Step one: Charge battery over night (assuming you do not own a 30 amp+ charger)
Test battery voltage at the battery posts, yes it is located in the &%#@ trunk. Clean the battery posts while you in there. Clean the battery ground cable connects too. Remove and clean the positive jump point under the hood. NOTE: disconnect the negative cable at the battery while doing all the cleaning mentioned.
Q? Reads over 12.4v? Proceed to step 2-4
Step 2: Read battery voltage with the engine at idle for 15/20 secs
Step 3: Read voltage at 1500 rpm same time
Step 4: Read voltage at 2500 Rpm same time

You are looking for fluctuating voltage at any rpm. Vehicle should display 13.2-14.7 constant voltage. And drop while the engine running is a volt reg going bad as the contact internally is lost (brushes). Turning on a few accessories will help pin any volt drops.

Replace alt with a new unit not a parts store reman unit. See one of vendors like ECS etc for quality units. Anything over 10 years is on borrowed time.
There are lots of postings on rebuilding your current unit if you have the DIY skills and time.

Helpful?

I refurbished both mine and wife’s alternators. The most often failure is the brushes supplying power to the armature via the slip rings.

If you are even remotely handy and can solder you can do a brushes only refurbish in about 2 hours including removing and installing the alternator.

The alternator maintains constant voltage output by changing the voltage to the armature. The primary symptom of failing slip ring/brushes is voltage fluctuations primarily at idle.

The lower the RPM, the higher the current requirement at the armature and the more susceptible to high resistance problems.

If you gave the valeo model, about $20 gets you new slip rings and brushes.

You have to pull the bearing to replace the slip rings so on my wife’s I just replaced the brushes, but since one slip ring wears 4-5x as fast as the other, I reversed the polarity to the armature so the more worn slip ring will stop wearing and the almost new looking slip ring will start taking all the abuse

The alternator is needing to be replaced. The intermittent function is now 11.1 volts.
"I can get 4 more miles out of this before I put it in the garage" I say. Nope. Got 2 more miles dropping off 95 year old grandma at the senior living place and walked 2 miles home. Do I need to use a code saver when I disconnect the battery on this model?
Me and my AAA friend Larry (or Ben or Stan or...) I'm almost on a first name basis with all of them, will be meeting there and bringing the X5 home this afternoon after church.

On some engine models like 3.0d you can remove the voltage regulator with brushes without removing the alternator. So changing the brushes is the easiest and cheapest way to do and usually get another 100k with that alternator.

On the 3.0 gas I considered trying that. The cap on the back has I think four screws that’s will be pretty tricky to remove but I never tried access from below with the stiffening plate off.

It’s actually not very difficult to remove the alternator, remove the fan & shroud to give yourself working room.

One more question (so far).
Is there any requirement to reset anything after disconnecting the battery before pulling the alternator?
On my 2002 there wasn't and anything e21, e30, and e34 I've had after that only required the radio code. I skipped e36 and jumped to e46, e53 and e90.