Hello everyone!

A while back i bought a BMW 520i e60 04 (lame model).
everything was fine the first few months but i have been having som issues with battery and charging. My battery went flat and disscharged over night after charging it so i went out and got a new one. Then all was good for 2 months. Now it has discharged 2 times and itīs really anoying.

Anyway, i charged the battery fully and used the OBD voltage reader (menu no.9). at idle it seems to be spot on sitting between 12.5-12.7. however once i turn it over it goes to about 13,7-13,8 it stays there for about a 30 seconds but slow starts dropping until it reaches about
12.6-12,7. Sometimes it dosenīt even go to 13,7 when i start it, it just stays at around 12,7ish. Seems like som intermitten operation. Is the running voltage to low?
I have read alot online but never seem to find a solutions on this topic. reving does not change the voltage. Altenator dead? voltageregulator? I would love to get some insight from you guys since Iīm not really an expert on the E60. (hope the grammar isnīt to bad)