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Flooded batteries are most vulnerable to freezing over the winter: in part because their electrolyte is more liquid, but mostly because they lose their charge far more rapidly that gel or AGM and the lost charge makes it more vulnerable. My experience is with gels primarily.
I try, and have always succeeded, in always having a “caretaker” for Alchemy over a winter’s storage. Often this is a fellow cruiser who will be in the area, sometimes a yard worker, sometimes a retired person familiar with boats. Their job varies but is primarily to charge the batteries at the schedule I set up ahead of time. I make things as simple as possible: usually the work is to just plug the shore power cord in (I never leave the boat plugged in 24/7).
If I think it is unlikely that there will be a hard freeze, I am comfortable leaving my gels fully charged for a few months unattended, but since I am looking at a 4-5-month lay-up, I usually have the batteries charged for a few hours on a monthly basis.
Removing your battery bank has always seemed to me a boat yard’s ploy to make a lot of money. Possibly it makes sense (in the old days) for flooded batteries in very cold climate where one is unable to find reliable caretaking. But then, you have to find someone reliable to watch over them (a bit) wherever they end up being stored. (I would not recommend attaching to a “trickle” charger and forgetting for the winter: a practice I suspect many boatyards indulge in: charge and then leave for a period and checking the voltage periodically). Over decades of storing boats in the company of many others, I do not remember anyone removing their batteries.
I do not disconnect my battery bank completely, although if leaving the boat unattended in a warmish climate and the batteries are not flooded, this may make sense as there are always small draws that eventually will bring batteries down. With reliable caretaking, I do not see disconnecting the batteries as necessary. I also like to have the bilge pump active as water will get in, in rainy climates, through the mast and then accumulate with the anti-freeze I have left sloshed in the bilge and can be then pumped out.
AGM’s sound like the battery to have in very cold weather. What was the resting v reading on your AGM’s after a winter on the hard in Greenland (or Iceland)? I am not sure gel’s are that robust with respect to cold and flooded batteries certainly are not.
I am sure I left something out: come back with questions/comments.
My best, Dick Stevenson, s/v Alchemy