Yup. I have a 2004 Z4 as well, ans it sits outside with a car cover all Winter long from early December until Mid-March usually. I try to get a battery tender on it at least once during that period. It's started right up every Spring except for last year, when the battery was almost five years old. I figure five years is a reasonable life for a battery, so I replaced the battery with an Interstate that was a perfect fit.

What?! No Optima? ;)

I didn't see an Optima battery that would fit with more than the 90 AH of the BMW OEM. What model would you propose as an improvement over the BMW AGM battery?

Not to mention that the Optima weighs more.