There have been a couple posts discussing proper battery charging and car storage. I'll throw a couple sense in here if I may:
"Cold" storage of the battery is not good. Low temps accelerate the creation of salts in the bottom of the battery as it discharges. Same stuff you learned in high school chemistry, or perhaps at home making rock candy with sugar and string. Keep the battery warm and you'll be able to keep the battery longer. Incidentally, this is the basis of the rule about never storing the battery on a concrete surface. Cold concrete = cold battery.
If the battery discharges sufficiently, the electrolyte can actually freeze. Like water freezing in your pipes, it will expand slightly. Enough to warp and crack internals. Sometimes just enough to crack the battery cases. No problem until the first thaw, when the electrolyte flows out through the crack.
The battery sits in a well in the rear of your shark under the spare tire. As a battery charges, it may cause hydrogen and oxygen gasses to accumulate in this closed area. One little spark is all it takes to get an interesting kaboom. I've seen the results of a few battery explosions, and they are not pretty. The harder the battery charges, the faster the evolution of explosive gasses, by the way.
Many chargers are not as "automatic" as others. Some will charge at a high rate to get the battery up to charge over night, then fall back to a 'trickle' rate of one amp or so. Owners may find that even the one amp charge rate is sufficient to cause electrolyte losses, especially when left attached for months unattended. While handy for keeping the battery warm, the loss of water from the electrolyte accelerates the formation of those deadly salt crystals.
Extended charging may eventually lead to terminal voltages in excess of 16 volts, a critical point above which survival of electronic components is compromised. Later cars with EZK and LH brains are particularly sensitive to overvoltage conditions. At around $1k for the part plus the hair removal exercise that precedes replacement, it's safe to say that this should be avoided if possible.
All this points to a reasonable battery storage solution: Remove the battery from the car while the car is stored. Place the battery on a shelf in a non-freezing location, in a spot where you'll be able to visit it once a month or so to check the electrolyte levels. Swing by your favorite Wal-Mart (or whatever...) store and grab a battery maintainer from the
battery-and-chargers display. The one you are looking for is about the size of a couple cigarette packs, made by Schumacher, and displayed in a hanging plastic display package. It's a model SE-112S when you get to the fine print on the label. It's about $25 or so last time I looked. It comes with two wires to attach to the battery connections, and a really short AC cord to plug in the wall. You'll need an extension cord. Anyway, hook up the wires, and physically attach the charger to the battery so the charger keeps that battery warm too. Comes with a couple metal bracket things, but a good bungee does the job for me.
The charger itself cycles power to the battery by watching the terminal voltage. When it drops to 13 volts, the charger turns on. Voltage rises to 14 as the battery charges, at which time the charger turns off. A couple status lights in the charger let you know that it's working OK and the battery is charged.
When you put the battery back in the car in the spring, take a little time to thoroughly clean all the cable connection points, including the ground point at the rear tray. Buff up the wing nut as appropriate to get good solid connections. Poor connections mean that not only will the car be starting a little slower, but that the alternator will get to make more voltage without all the battery to restrain the peaks. I won't get into the technical part of how the alternator makes AC and all, just make sure the cables and battery connections are solid. Avoid that 'potentially' nasty situation with overvoltage spikes on those expensive electronics.
From sunny Southern California, where I keep the batteries from the boats and race cars on a charging rack during the frigid winters, and drive the shark on days when it doesn't rain... (sorry...)
NOTE trying to start a 928 with a weak battery can damage the relays,
What happens is that the relays do not get a solid 12V closing signal,
this then makes the contacts have less closing force,
this then creates an arcing situation,
the arcing makes the contacts dirty,
the dirty contacts no longer pass the same amount of current.
the part that is controlled by the relay no longer gets a solid 12 V.
NOTE dont try to start a 928 with a weak battery,
always charge the battery with a battery charger first,
6 amp charging for 4 hours is a good start.
NOTE a trickle charger is not a battery charger.
NOTE charge the battery every 2 months if the car is not used frequently.
NOTE disconnect the battery from the car before charging.
NOTE disconnect the battery if the car will not be used for two or more weeks.
NOTE don't jumpstart the car with a weak battery,
damage to the relays , computers and charging system can result.