Well, you could go to the dealer, but it would be expensive.

All you need is at least a CCA rating of 720, and an AH rating of 90. It needs to be the right size, and to have the vent on the correct side (since it is mounted under the rear floor). Any auto parts store will have a chart that shows what battery that they carry should fit your year and model. Check the CCA and AH ratings, and the vent location, and you are good to go.

Here is a tutorial for you on changing it:

http://www.xoutpost.com/bmw-sav-foru...apped-out.html

If you want specific brand recommendations, suggest you do a search. A quick search of just the E53 forum for "battery replacement" got me 268 hits.

Jeff

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue