Different battery technology and winter circumstances dictate an individualized regimen. Always charge batteries fully before leaving them unattended. When you layup batteries for any length of time beyond a few weeks leave some sort of charger attached which will keep up with battery self-discharge. Ideally the battery voltage should be maintained between 13.2 and 13.6 Volts. A small solar panel is often sufficient to maintain a battery.
Solar panels can overcharge, however, so be sure to use a regulator on larger panels.
If you can't leave a charger attached, apply a full charge every 3-4 weeks. Basically, a full charge is a process where a temperature corrected absorption voltage is applied to the batteries until battery current declines to a low percentage of battery Amp-hour capacity. Batteries which are fully charged won't freeze in weather typical of the U.S., except perhaps in Alaska. Batteries that are not fully charged may freeze, and the expansion of the ice will probably fracture the cases. As you may know there are four distinct types of lead-acid batteries.