If you've got a Costco membership, their pricing and warranty for Interstate batteries is fantastic. Dead battery under warranty? Just return it and buy a new one with the refund. No hassle. No testing. Too bad that the X5 is the one oddball in the collection (aside from my truck) that doesn't take an H6.

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2011 M3
2006 Sierra 2500HD 4WD LBZ/Allison
2004 X5 3.0i 6MT
1995 M3 S50B32
1990 325is
1989 M3 S54B32

Hers:
1989 325iX
1996 911 Turbo