12.6 VDC is basically a fully charged battery or nearly so. The myth about leaving the battery on a cement or concrete floor is no longer valid. It was from the days when the battery casings were not waterproof and hasn't been applicable in a long, long time.
A hundred years ago when battery cases were made of porous materials such as tar-lined wood boxes, so storing batteries on concrete floor would accelerate their discharge. Modern battery cases are made of polypropylene or hard rubber. These cases seal better, so external leakage-causing discharge is no longer a problem, provided the top of the battery is clean. Temperature stratification within very large batteries could accelerate their internal “leakage” or self-discharge if the battery is sitting on an extremely cold floor in a warm room or is installed in a submarine.
Hopefully, the batteries aren't being stored in a really warm room with a cold concrete floor, which would lead to greater self-discharge due to the temperature in any case....Also, most batteries used by small sailboat owners aren't large enough for temperature stratification to be an issue in any case. If the top of the battery is clean, it doesn't matter that condensation forms on it... relatively pure water, which is what condensation is, is a lousy electrical conductor.
If you leave them aboard you'll need an occasional recharge to keep them above 12.6 volts. There's little chance of freezing the battery if it's maintained above 75%. I'm not sure about the need to remove the negative lead, but it could lessen the potential of a short circuit in any part of the electrical system causing a fire.
A battery will slowly bleed it's charge if left on board. Temperature changes over the winter will allow a thin film of condensation to form on the battery case. This condensation will conduct a small trickle of current between the battery posts. Every month or two you'll need to top up the charge.
If possible, consider removing the battery and storing it at home in a dry cool spot. Never leave it on a cement floor as the cool cement will allow the same layer of condensation to form from minor air temperature changes. Even a small piece of plywood will insulate a battery from the cement.
A battery could last for five to ten years if properly maintained. Keep it charged, clean and dry. Batteries don't like to be quickly discharged or charged as excessive heat may cause internal plates to warp and touch. Prolonged cranking of a hard starting engine will shorten battery life. Charging a fully discharged battery with the engine will reduce it's life as the alternator will be hitting it with a lot of current for the full charge period.
When stored on the hard I don't need a bilge pump as I have two gabboard drain plugs. I'd rather allow the water to continually drain out than risk the high current draw of a frozen bilge pump.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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