I agree, make sure that it is the same width. you want to be able to lock it down. I just replaced my battery with one from Autozone. The first one they gave me was too narrow and I could not lock it down. I went back and got the wider one (H5 vs. H6). I understand that BMW used several different battery sizes. The amp hrs and cold cranking amps can vary, more is better.

The vent tube is critical. You don't want battery fumes or hydrogen building up in your trunk and cabin.

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2008 Space Gray Z4 3.0 si, Saddle Leather interior
2011 Nissan Frontier King Cab
2010 Lexus RX350