A battery will slowly self-discharge unless the engine is started periodically and run for a few minutes, or some sort of trickle charger is used to maintain it. Discharged batteries can freeze, but it's almost impossible to freeze a fully-charged one. The best possible situation would be to store them in an area that does not go below freezing, and use a trickle charger or a battery maintainer to keep them charged.
Even if it does not freeze, a discharged battery that stays that way for several weeks will develop hard crystals of lead sulfate on the plates, and it will be extremely difficult to recharge. Keeping a battery fully charged at all times is the best way to assure it has a long, useful lifespan. Rule #2 is keep it bolted down- - - -bouncing around destroys a battery in a hurry.
Jerry
