Long-term storage. Need advice...

MacCool

In the op's case it may be moot because he mentioned that he thought the battery might already be shot, but the advice above is off the mark.

• Disconnecting the battery is unnecessary if the car can be stored with a float charger like "Battery Tender" attached
• Storing a modern battery on a concrete floor isn't a problem. That's an old wive's tale that hasn't been true for almost 20 years - certainly since they've been using polypropylene cases.
• Cold weather is not a problem for a charged battery. Electrolyte in a fully charged battery won't freeze until about -60 degrees F. Another reason to keep the battery on a float charger.
• If you can't leave the car plugged in or don't want to, it's reasonable to pull the battery and store it somewhere on a float charger, or to just pull the battery and scrap it, accepting its replacement at the end of a year of car storage as part of the cost.

The thing that causes the demise of any lead-acid battery is primarily sulfation---lead sulfate bridging of the plates causing internal shorts, higher resistance, inability to take a charge and an increased rate of self-discharge. The thing in turn that causes sulfation primarily is partial discharge or repeated charge/discharge cycles. Sulfation begins anywhere less that a full charge, but is particularly enhanced by a charge state less than 80%. Store your car for the winter without a float charger on the battery, even if you go to the unnecessary step of disconnecting it or removing it from the car, and you are shortening its lifespan to a huge degree.