I think you have a grounding cable issue... make sure all cables are clean and tight... what is the voltage reading on the battery? You could swap the battery, just to make sure, but I had issues with starting my 4.6is after I accidently drained the battery cleaning the interior... After charging for a couple hours and it failed to start, I turned off the X, set the alarm, let it be for a while, then tried it again... alarm off, turned to "on" and crossed fingers... hit the starter and it worked... not sure if fingers need to be crossed! That step might be optional!

Good luck!

__________________
2001 E53 X5 4.4i (03-07-2001 build date) White exterior / hellbeige interior (6MT swap vehicle - Manny)
2002 E53 X5 4.6is (12-20-2001 build date) Imola Red exterior / M-texture interior (Red)
2003 E53 X5 4.4i (06-26-2003 build date) Silver exterior / Black interior (Silvester II)
2008 Mini Cooper S ragtop (04-17-2008 build date) Dark Silver exterior / Gray interior ( Topless)
BMWCCA member (#4745)