appreciate all the replies, seems general consensus is let the oem old battery go, and either way temps are now down to 15c and car started fine with new bosch , whereas at a temp of 19c it would not start up with the old oem battery.....temps rarely go below 5c in winter in the coastal regions...but am planning on some hitting the mountains and snow, so will be interesting to see how it holds up...again thanks for all the replies...last note....what out of all the items on x5 would drain most current? pano sunroof? xenons? window lifters? or the window heater at rear?or the horn (i killed the battery on my merc after over enthusiastically honking all night long after a soccer match win)

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E53 X5 3.0d SUV prod.date 11/2004 eng.variant M57N Europe Right hand drive N