You didn't say what kind of batteries you have.
Gelled and AGM types have much lower self-discharge rates than do flooded batteries. While you might get away with fully charging the first two types and leaving them over the winter, I wouldn't do that. Much better to keep them on charge, with an appropriate smart charger.
For flooded batteries, you really do want to keep them on a charge, either in the boat or at home. Again, you need a smart charger which will float the batteries at an appropriate voltage level.
Flooded batteries left in a boat over the winter can freeze if they aren't well charged. You don't want that.
My batts have not left the boat in 20 years+. Perhaps because when I was young my folks lived in Alaska and I saw batteries operate at temps than can freeze spit before it lands that I am not inclined to buy into the "dread and fear" of cold storing batts.
While they don't have a lot of "capacity" or CA at those temps they do last a good long while.
Cold weather actually benefits and can extend the life of lead acid batteries. It dramatically slows self discharge and can also slow the sulfation process way down as the self discharge is considerably slower in colder temps. While most manufacturers don't recommend storing at temps below 32F when I questioned a few, Crown, US Battery & East Penn they admitted this is because people often attempt to store discharged batteries and then they try to charge them frozen so it is more of a "liability" reason than scientific.
The batteries must obviously be topped up to prevent freezing and if they are they can be good for a couple months in-between charges, if the winter temps stay cold. They should also be completely disconnected so parasitic loads don't kill them. A fully charged battery won't freeze until -72. In the last 20 years we have had winters with down to -30 and my batteries were fine.
Last winter I actually paid attention to this. I charged them to full in early November and purposely did not put a charge on them again until early April just to see the effect cold had on self discharge. The batteries were still at a resting open circuit voltage of 12.68 volts and this was confirmed with an SG test and the same DVM I had used in the fall. Full on this bank is about 12.73V. If they had been in my basement I would have needed to top them up every three to four weeks due to the much higher temps and accelerated self discharge.
I have a bunch of deep cycle marine batteries sitting around my shop. In the summer I need to flip the charger on about once every two to three weeks to keep the batts at full charge. In the winter I rarely flip it on as they just don't self discharge at anywhere near the rate they do in warmer temps. I purposely store them in my unheated shed not in my heated basement or garage..
Like you I do test my batteries, load, calculated discharge, capacitance and pulsed load testing, and get tremendous life out of ordinary cheap "deep cycle" wet cells despite storing them in cold temps and never float or trickle charging them. Leaving them off charge for multiple months last winter I thought for sure I wold see some stratification but the SG tests did not show it. Course I drain & re-fill the hydrometer three times on each cell so that may have a "mixing effect" that helps stir up the electrolyte in each cell? Ideally I think they should be topped up a few times over the winter if for no other reason to get the electrolyte rolling..
I don't trickle or float, other than solar in the summer, and still will likely easily see 7+ years out of a $210.00 375 Ah bank.. Ending year five in a few weeks and performance is still exceptional under all tests. This spring they put up about 98% of new Ah capacity in both a 25A load test to 10.5v and a 20 hour load test to 10.5V... Two days ago the bank put up 2486 CCA as measured with my Argus 500. This was actually within 3 CCA of where it had been in the spring and 3CA higher.. Of course that 3 CCA is well within any margin of error and that is what I expect it is not that the batts got healthier. While I prefer the Midtronics capacitance testers the Argus is very repeatable.. (Note: these testers do not test for Ah capacity just cranking capacity changes)
I have become a strong believer that my banks actually have better health because they are cold stored and not constantly floated. Was speaking with an East Penn engineer a few months ago and he said they actually advise against constant trickle charging and sent me this (the bolding is theirs):
leave a battery on a trickle charger longer than
48 hours. Serious damage to the battery WILL occur. "
Lifeline Battery also advise against this..
Of course this is an entirely controversial debate in and of itself with no clear conclusion I have been able to decipher.
All I know is that my customers in marina's who charge 24/7 often have worse outcomes, on average, than do my customers who use solar to keep the batts up. As my solar customer base grows perhaps this will even out.. I suspect "proper" float charging is fine but I also suspect most people are not "properly" floating. For example a very small % of the chargers I work on that have temp compensation are actually using it...
In mid October I had to replace a bank of T105's that were only 4.5 seasons old. Very few cycles, guy hardly uses his boat, but when I looked into the cells the plates looked spongy and eaten away. Had been floated with a Xantrex Freedom and the charge profile was "correct" based on what the Xantrex manual states.. This is not an uncommon event I see with my dock side customers who tie up and plug in..
I suspect, like you've found, many of these chargers are floating too low... My Genasun MPPT solar controller floats at 13.8V but only for 4-5 hours per day, if that, depending upon weather.. ...