I think your battery is screwy. Put the old one back in or another one in and see if the problem disappears.

I've seen a lot of cars where the battery and alternator both go around the same time for some reason.

Check the voltages with the procedure below:

To UNLOCK the OBC:
Turn ignition to position 1 (position 2 won't work unless you FASTEN YOUR SEATBELT)
Push right button in the instrument cluster and keep it pushed. You will first see "Check Control OK", some seconds later you will see "Test Nr.01"
Push the left button on the instrument cluster to see the car's identification number. Add the 5 digits together and write down that number, you will need it for unlocking all the functions. Example: DG23278 will lead to 2+3+2+7+8 = 22
Push the right button until you see "Test Nr.19"
Push left button until you see the number that is equal to the one you calculated above
Push right button again

Now you will have unrestricted access to Test No.3-21. If error in procedure, Test No.3-21 will only display Lock:ON

Most of the tests are self-explanatory / What you will see is below each test:
Display Car's identification number, HW/SW version, construction date (week/year)
Several numbers and data
A very beautiful instrument test
[Instrument Cluster Display Test]
Current consumption (liters/hour)
VBR: x,x (liters/hr)
Average consumption (liters/100 km)
RW-VBR: x,x (liters/100 km)
Current range
RW: x (km)
I think this is remaining fuel in tank. Three sets of numbers: A/B/C. And I've noticed A+B=C.
T: (liters)
Engine RPM / Coolant Temp (C)
TMTL: x,x (liters) / KTMP C
Current Speed (kph)
V: x (kph)
System voltage at Term. "R" (shows Battery Voltage w/engine NOT running...shows Alternator w/engine running)
UB: xx,xx (V) This shows voltage
Country/Language (US, Spa, Ger, Jap, etc.)
LAND: 2=USA, 3=I, 4=E, 5=J, 6=F, 7=CDN, 8=AUS/GOLF, 0=D, 1=GB
Units am/pm
EINHEIT 1: B3, B4, B5, B6, B7, B8, B9, BA, BB, BC, BD, BE, BF, B0, B1 EINHEIT 2: [similar to above, I didn't want to mess with it]
Average Speed
VANK: xx,x (kph)
ETA
ANK:xx:xx
Date of Software Mask
ROM:
Production Diagnosis
DIAG: 01 01 000 E3
(may say OEL TEMP...but numerical digit remains constant regardless of oil temperature)
OEL TEMP: -48 C
Display Vehicle Specific Data
PROM: 00 12, 01 37, 02 18, 03 E6, 04 02, 05 B3, 06 FF, 07 7E
Alarm Changeover (cont. vs. intermittent OBC chime)
HORN: DTON or [DTON flashing]
Lock/Unlock
LOCK: ON or LOCK: OFF
Correction factor for fuel consumption (nicht ändern!)
KVBR: 1000
Reset all defect codes
RESET?