During charging, the process reverses itself. Therefore, if a battery is left in a discharged state for a long period of time, the damage can be irreversible. If you run a battery down, it's important to charge it as soon as possible and not wait until the day before you go fishing. The opposite, overcharged, can be even worse. This causes battery acid to percolate, which can then overflow or escape as gas through the vent, with resulting heat buildup, the plates will soften and shed the charging grid's active material, causing a dramatic decrease in productivity.
Cold temperatures are ideal for storing batteries.
Heat accelerates discharge. A battery will retain its charge indefinitely at temperatures less than 37 degrees F. so leaving it our for the winter isn't a problem. The important thing is to make certain it's fully charged, because a deeply discharged battery's acid lowers a specific gravity similar to water and can freeze at temperatures near 10 degrees F. Battery placement is another concern. Most sterndrive boats have the batteries in the engine compartment, which is the equivalent of sending them to Death Valley for the summer. Boaters can counter extreme compartment heat by partitioning off the battery compartment with styrofoam.