FYI, this has been covered before, but here is the OEM information on the recommended BMW-branded C-TEK battery tender.
I'm using it during the 3 winter months my M2 sits in an unheated single garage.
I've hooked it up today to my 2012 328i as this extreme cold spell is now causing it to display 'low battery'. Hopefully the battery tender will be able to salvage it, otherwise I'll have to buy a new battery.
https://www.shopbmwusa.com/PRODUCT/4...ATTERY-CHARGER
$140.00
https://smartercharger.com/battery-c...K Multi US 4.3
$88.99
https://www.amazon.com/CTEK-56-864-A...2%3A2661618011
$66.26
You may also want to purchase the extension cable if you wish to extend the connections.
https://www.amazon.com/CTEK-56-304-C...xtension+Cable
$12.25