Usually I store my batteries in the basement over the winter, November through March. No charging and the voltage at the end of March is well over the 12 V minimum.
This year I disconnected my batteries for the winter, leaving them in the RV. I put a maintainer on them for one day in January or February since it can be quite cold in Northern Alberta.
This is the maintainer I used.
http://www.ebay.ca/itm/NEW-car-Motorcycle-ATV-12V-Smart-Compact-mini-Battery-Charger-Tender-Maintainer-/271618074547?hash=item3f3db2e7b3
This $5 gizmo provides up to 0.7 amps and reduces the voltage as the battery nears full charge.
One problem: the power cord cracks and fails in cold weather.
