I replaced mine earlier this year with a Duracell from Batteries Plus. I don't recall the exact cost, but it was in the $120 range with core return, and I installed it. That replaced the original too. 12 years isn't a bad run for a battery, but you've got to admit, BMW gave the batteries a pretty cushy spot in the X5 (and many others.)

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2001 X5 Sport 3.0/5-speed
1998 318ti/5-speed
1988 735i/5-speed
1984 528e/5-speed (soon to be M20B25-powered 525i!)