Seems like the battery/charging issue is an ongoing even in the E70's gauging by a few of the posts on there.
I wonder how many people have forked out for a new battery, alternator or voltage regulator & still had the same problem.
The battery I replaced is still fine & I have re-installed it and will try and sell the new unit and get some money back.
BMW have quoted >$400 for a new voltage regulator & $1400 for a re-furbished alternator !!!!
From what I have read on here neither of them will guarantee to fix this issue so in the interim I will look at getting a cheap volt meter wired in and avoid stopping when volts are low - beware of opening doors, radio on etc before you take your car out for a run - if you just go down to the shop for fuel or anything & stop the engine it may not start if the alternator hasnt had a chance to put some charge back into the battery & if the charge is very low even a jump start wont get you home, it could stop dead in traffic!!!
The voltage on my X sometimes drops to 10 or lower (depending on the charge you have to start with it can take ~45 mins before the alternator starts charging), if you stop the engine at that level it's unlikely to start again.
Cheers & beware......... Im not sure what its going to take to fix this issue but Im definitely not parting with any more money until Im confident of a permanent fix to the issue.

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X5 4.4i 2006 sports package, Titanium silver, Sports exhaust with Xpipe, Limo tint, 20" wheels 168's (Black), 25mm H&R wheel spacers& arch flares all round - Loud........

Work in progress -:
Replace BMW Ipod system with DICE - installed but no sound yet??