I second the Interstate nomination.... (archive)

Used one in a friend's M3, my 535, and now my own M3 (just last week). The size I got was MTP-91, 750CCA, 72 month warranty. About $100 from an Interstate dealer. If you have a big stereo, see if you can find the larger model. I don't remember the part number (maybe MB-93?). The MTP-91 is about 3" shorter in length (and several pounds lighter). The battery box has two different holes to bolt in the clamp, so no mods are necessary.

Also, just starting it once a week may not be enough to keep the battery up. Drive it to the store or somewhere for more than 10 minutes. The cold temps will drain the battery pretty badly, and idling for a few minutes will not restore the lost charge. If there's snow everywhere (where do you live?) then consider buying a batter tender. They maintain a constant charge in the battery and will not overcharge it.

James
88 M5
88 M3
83 535i

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