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From what your saying sounds like the battery's had its day, not long after I got mine although it started first time every time is noticed that upon unlocking my day time running lights weren't functioning as it was winter also the rear window demister wasn't operating. I'm sure my battery was the original item the eyelet was black my cars a late 2006 meaning it had lasted 6 years, which isn't bad IMP they are good for 4-5 years if used on short journeys perhaps less than 4!
When I checked sure enough my battery(drop tested + diagnostic checked with all system going) was a bit lower than I'd liked to of seen on voltage and within 24 hrs the magic discharge light popped up in the dash for the first time since I bought the car and via diagnostic a further drop had taken place since the last check, so that was it as far as I was concerned.
Changed to a nice Bosch S5 battery registered without drama everything restored. Being a std white battery mine was a non AGM setup, which yours I suspect will be being a 2007 car, therefore battery costs aren't too horrific, but check your battery re colour as I could be wrong but sounds like you've got a non AGM battery with the age of car you've got. Euro's GSF will do the battery's you'll need.
The cars priory aim in life re power, is to ensure it runs stops and functions as designed but with power issues it will shut things down and run with a basics if necessary if powers not acceptable with the battery for instance it shuts down certain functions/control units to ensure power is available to maintain the basics you don't need heated rear windows, mirrors, sat nav, phone to drive he car therefore if powers needed to ensure the car starts, drives as it should then it'll save power by closing these secondary item down.
The cars intelligent battery system/IBS cable that Kris points out is quite critical in all matters regarding electrical power distribution and will detect drain but these units as again Kris can confirm may/can fail as well (this will show up via a diagnostic check with a decent BMW Indy/Main Dealer)especially on some of the earlier cars.
We do a shed load of battery's at our place plus a few IBS cable replacements when the IBS statement is highlighted via diagnostic as having as issue. Later spec cars for instance come in to us people report various issues including non functioning stop/start again when checked you see codes re under voltage battery's changed everything works these AGM battery's IMP on the later cars are good for 3-4 years.
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