One of the fall rituals observed in a boatyard where I once worked involved the removal of batteries from most of the boats that were wintering over. The batteries were placed in a heated shed, paralleled and charged periodically. The logic was that because batteries always seem to have less power when cold, keeping them warm during the winter must be better for them.
In hindsight, this was a less than ideal approach. The potential for damage to the battery and the boat—not to mention the backs of the poor souls doing the lifting—was very real, and, as it happens, flooded and sealed valve-regulated lead-acid (SVRLA) batteries endure extreme cold just fine.
In fact, the reason a battery has less oomph when it’s cold is because the falling temperature is slowing down the battery’s internal chemistry, sending it into a sort of hibernation. This is actually desirable when a battery is not being used, with one provision: It must remain charged. A fully charged battery won’t freeze, while a dead or weak one will.
If a battery is allowed to freeze, it will almost certainly suffer internal, and possibly external, damage that may result in a fire or even an explosion if a charge is applied while the battery is still frozen, or even after it thaws. The guideline here is: Leave batteries aboard, make certain all loads are disconnected and charge periodically (once a month is probably enough for flooded batteries, every three months for SVRLAs).