It needs to be load tested. If they just put a volt meter on the battery that mean nothing. If you ever had a "Stress" test to determine the condition of your heart you know that they just don't take your blood pressure and call it a day. They get your heart rate up so they can see what the heart is doing under stress.

Are you taking the battery out of the back trunk when you go to get it tested or do they just hook up their tester to the battery while it is still in the trunk?

Or worse, are they using the terminals under the hood?

They did load test it. Well at least they said they did. The first time they hooked it up to the machine while it was still in the trunk. The second time I took the battery completely out of the car and took it to them separately. Both times they load tested it and the second time they also tested it to hold a charge. So unless they lied twice or their equipment is bad or wrong it's gotta be the alternator?

Battery testing 101
instructions attached in PDF form and a quickie chart to leverage a volt meter. Without a carbon pile load tester (see image below) everything else is a SWAG. The cheapie hand held devices can not load a battery to the correct levels, they just load something (What it is the operator can not tell you).

Wet cell auto batteries are load tested at 50% of their cold cranking AMPS ratings. If the voltage fall below 9.6 volts the battery is bad or undercharged. Recharge at 30 amps for 15 minutes and retest. Period end of story, and has not changed in 30 years. Trust me on this one as I sold the damn things for years to Sears, Good Year, Penske, Firestone, BMW, Mercedes, GM, FORD, you name it. If they build or worked on cars this is what they use.

Batteries when they croak, tend to hold only a surface charge (Hint: read 12V or less) but drop off to below 9.6V as soon as they are load test regardless of charging frequencies or charging amp draw. If you ever take one apart (NOTE: FULL OF ACID, DO NOT OPEN IN or ON ANYTHING YOU WANT TO KEEP AFTERWARDS), you will find the cell walls are touching hence only a few cells develop voltage. You see it as the car cranks slowly but fires up. If you turn on the headlamps FIRST then crank it will just click.

Alternators: Test and replace as the brushes in the regulator wear out after 100K or faster depending on the load. Load testing an alternator is done at the battery with the same meter shown below. Bring the engine to 1500-2000 RPM and load the system until you see 12 volts, note the alternator AMP output. It either provides steady output or not. Read diodes- Pass/fail. That is it. You can then do bench repairs as a DIY but at a shop it is a remove and replace task. No one bench tests anymore. Cheapie rebuilts have their own sad story that has been posted before.
Note: LOTS of energy usage equals faster wear. Drive at night? Drive with all the accessories on? High demand = short life. Our high end heavy demand X5s EAT batteries and have a high demand on the alternators. Fact of life.

These vehicles are energy hogs. Run great and drive like the wind though:D

Sorry to get so long winded on this topic.....

When your car battery goes dead overnight
 

Finding the hidden draw on BMWs... Not mine but a great approach and well thought out.

When your car battery goes dead overnight, usually either the battery is at the end of its life span, or you left something on, such as a light. Occasionally something is drawing power that's not of your doing. This is a parasitic draw, and it can cause the same result as leaving the headlights on: a dead battery in the morning.

1. Remove the negative side battery cable from the negative battery terminal.
2. Connect the black wire to the com input on the multimeter and the red wire to the 10A or 20A input on the multimeter. The meter needs to be able to read at least a 2 or 3 amps for this test to work. Connecting the red wire to the mA input on the multimeter won't work and could damage the meter.
3. Attach a multimeter (set the dial on the multimeter to measure Amps as per multimeter instructions) between the negative cable and the negative battery post. Wait a few seconds to several minutes for the car to go into sleep mode - i.e. when you make the contact with the ammeter, the cars computer systems "wake up". After a bit of time they will go back to "sleep".
4. If the ammeter is reading over 25-50 milliamps, something is using too much battery power.
5. Go to the fuse panel(s) and remove fuses, one at a time. Pull the main fuses (higher amp ratings) last. Perform the same steps for relays found in the fuse panel. Sometimes relay contacts can fail to release causing a drain. Be sure to observe the ammeter after pulling each fuse or relay.
6. Watch for the ammeter to drop to acceptable drain. The fuse that reduces the drain is the draw. Consult the owners' manual or service manual to find what circuits are on that fuse.
7. Check each device (circuit) on that fuse. Stop each lamp, heater, etc. to find the drain.
8. Repeat steps 1 & 2 to test your repair. The ammeter will tell you the exact numbers.

Thank you for the very detailed posts @StephenVA! I will probably go get it tested between work and School today, the battery that is. Which will be within the next hour or so. And we shall see where that takes us.

On finding a parasitic drain, really thank you on that as well because I had absolutely no idea where to begin. I had an idea of how to do it but yours is so much easier!

I recently installed the 3rd battery in 14 years, average 7 years each. Never done any alternator work in that 116,000 miles. Air conditioning is on about 98% of the time, seldom has the radio been silent, have an aftermarket amp, meth pump is running much of the time, replaced the clutch fan with electric fan, list goes on. Fully loaded X5 and same electronics as other E53s. I got the only one that doesn't eat batteries and alternators?

Nope...U B "special".
The brushes are wearing away on that alternator as you drive....:bustingup.

I personally have driven a lightly loaded alternator 250K in 15 years. Just replaced bearings at 100K. Washed under the hood too often I guess. :dunno:

Some go fast some slow. Who knows?

Too much info Update: Voltage reads hi and low at different RPMs.

If the shaft of the rebuilt alternator has not been cleaned of carbon and ground (i.e. machined) smooth, the brushes will at the least prematurely wear or create a hi charge/ no charge state (on a voltmeter you will see 12.xx volts then a jump to 14.xx as there is a clean connection. REAL BAD for both the alternator and the regulator as it can not handle fluctuation for long. Usually this flux will take out the alternator/regulator quickly.

The same issue with cheapie regulators in an otherwise good alternator.

+1 on this.

Given you can verify and solve any parasitic drain issues, and are confident the battery is good, if you unlock the cluster and access the battery voltage menu (#9 on the updated clusters), watch for fluctuations while the car is running.

If the voltage dips and jumps around, away from the ~14.0-14.3V the alternator "should be" constantly putting out, you may suspect the issue is alternator related.

I had similar symptoms with an irrational battery, and solved all with a new voltage regulator.

Alrighty so sorry for the delayed reply. FINALLY had the chance to put the new alternator in. Everything seems to be fixed thankfully! No more red battery light for sure. Tonight/Tomorrow I will see if the battery is still draining. Fingers crossed!