John,
I use an old Battery Tender to keep my cars and M'cycles topped off:
this is what I used for my 5 month storage, in a cold garage:
Battery Tender Link:
http://batterytender.com/default.php...285fc05281880f

Some BMW Dlrs also sell the identical prod. with the BMW Roundel logo
and a cigarette lighter plug in, for about ~$80.

Here is thread with pics, from my winter storage process:
http://www.xoutpost.com/x5-e53-forum/...-term-car.html

GL,mD