Yup...beware of "maintenance free" sealed LIQUID batteries. They will absolutely need to be charged during the winter at least once every couple of months to a full charge.
Genuine AGM's or GELS can be fully charged and then left all winter though it would not hurt to check their state of charge once every couple of months with a voltmeter. You should not let it get below a 12.25 V reading.
I also question the wisdom of disconnecting the negative cable. All too often a leak can develope or water can enter from the mast or ice/debris and rain can clog the cockpit drains and make them over flow into the cabin. I would always want a working bilge pump even on the hard. Not nice to find a flooded cabin in the spring.
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